Adds execute script API (#1769)

* Adds execute script API

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Reword a sentence

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Punctuation

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

* Update _api-reference/script-apis/exec-script.md

Co-authored-by: Nate Bower <nbower@amazon.com>

* Update _api-reference/script-apis/exec-script.md

Co-authored-by: Nate Bower <nbower@amazon.com>

* Update _api-reference/script-apis/exec-script.md

Co-authored-by: Nate Bower <nbower@amazon.com>

* Update _api-reference/script-apis/exec-script.md

Co-authored-by: Nate Bower <nbower@amazon.com>

* Incorporated editorial comments

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>

Signed-off-by: Fanit Kolchina <kolchfa@amazon.com>
Co-authored-by: Nate Bower <nbower@amazon.com>

_api-reference/script-apis/exec-script.md

@@ -0,0 +1,182 @@
+---
+layout: default
+title: Execute Painless script
+parent: Script APIs
+nav_order: 7
+---
+
+## Execute Painless script
+
+The Execute Painless script API allows you to run a script that is not stored.
+
+### Path and HTTP methods
+
+```json
+GET /_scripts/painless/_execute
+POST /_scripts/painless/_execute
+```
+
+### Request fields
+
+| Field | Description | 
+:--- | :---
+| script | The script to run. Required|
+| context | A context for the script. Optional. Default is `painless_test`. |
+| context_setup | Specifies additional parameters for the context. Optional.| 
+
+#### Sample request
+
+The following request uses the default `painless_context` for the script:
+
+```json
+GET /_scripts/painless/_execute
+{
+  "script": {
+    "source": "(params.x + params.y)/ 2",
+    "params": {
+      "x": 80,
+      "y": 100
+    }
+  }
+}
+```
+
+#### Sample response
+
+The response contains the average of two script parameters:
+
+```json
+{
+  "result" : "90"
+}
+```
+
+### Response fields
+
+| Field | Description | 
+:--- | :--- 
+| result | The script result.|
+
+
+## Script contexts
+
+Choose different contexts to control the variables that are available to the script and the result's return type. The default context is `painless_test`.
+
+### Painless test context
+
+The `painless_test` context is the default script context that provides only the `params` variable to the script. The returned result is always converted to a string. See the preceding sample request for a usage example.
+
+### Filter context
+
+The `filter` context runs the script as if the script were inside a script query. You must provide a test document in the context. The `_source`, stored fields, and `_doc` variables will be available to the script.
+
+You can specify the following parameters for the filter context in the `context_setup`.
+
+Parameter | Description
+:--- | :---
+document | The document that is indexed in memory temporarily and available to the script.
+index | The name of the index that contains a mapping for the document.
+
+For example, first create an index with a mapping for a test document:
+
+```json
+PUT /testindex1
+{
+  "mappings": {
+    "properties": {
+      "grad": {
+        "type": "boolean"
+      },
+      "gpa": {
+        "type": "float"
+      }
+    }
+  }
+}
+```
+
+Run a script to determine if a student is eligible to graduate with honors:
+
+```json
+POST /_scripts/painless/_execute
+{
+  "script": {
+    "source": "doc['grad'].value == true && doc['gpa'].value >= params.min_honors_gpa",
+    "params": {
+      "min_honors_gpa": 3.5
+    }
+  },
+  "context": "filter",
+  "context_setup": {
+    "index": "testindex1",
+    "document": {
+      "grad": true,
+      "gpa": 3.79
+    }
+  }
+}
+```
+
+The response contains the result:
+
+```json
+{
+  "result" : true
+}
+```
+
+### Score context
+
+The `score` context runs a script as if the script were in a `script_score` function in a `function_score` query.
+
+You can specify the following parameters for the score context in the `context_setup`.
+
+Parameter | Description
+:--- | :---
+document | The document that is indexed in memory temporarily and available to the script.
+index | The name of the index that contains a mapping for the document.
+query | If the script uses the `_score` parameter, the query can specify to use the `_score` field to compute the score.
+
+For example, first create an index with a mapping for a test document:
+
+```json
+PUT /testindex1
+{
+  "mappings": {
+    "properties": {
+      "gpa_4_0": {
+        "type": "float"
+      }
+    }
+  }
+}
+```
+
+Run a script that converts a GPA on a 4.0 scale into a different scale that is provided as a parameter:
+
+```json
+POST /_scripts/painless/_execute
+{
+  "script": {
+    "source": "doc['gpa_4_0'].value * params.max_gpa / 4.0",
+    "params": {
+      "max_gpa": 5.0
+    }
+  },
+  "context": "score",
+  "context_setup": {
+    "index": "testindex1",
+    "document": {
+      "gpa_4_0": 3.5
+    }
+  }
+}
+```
+
+The response contains the result:
+
+```json
+{
+  "result" : 4.375
+}
+```