Add shard action

Signed-off-by: Naarcha-AWS <naarcha@amazon.com>

_im-plugin/ism/policies.md

@@ -99,6 +99,7 @@ ISM supports the following operations:
 - [read_only](#read_only)
 - [read_write](#read_write)
 - [replica_count](#replica_count)
+- [shrink](#shrink)
 - [close](#close)
 - [open](#open)
 - [delete](#delete)
@@ -162,6 +163,46 @@ Parameter | Description | Type | Required
 
 For information about setting replicas, see [Primary and replica shards]({{site.url}}{{site.baseurl}}/opensearch#primary-and-replica-shards).
 
+### shrink
+
+Allows you to reduce the number of primary shards in your indexes. With this action, you can specify:
+
+- the number of primary shards that the target index should contain.
+- A max shard size for the primary shards in the target index.
+- Specify a percentage to shrink the number of primary shards in the target index.
+
+```json
+"shrink": {
+    "num_new_shards": {
+        "type": "integer"
+    },
+    "max_shard_size": {
+        "type": "keyword"
+    },
+    "percentage_of_source_shards": {
+        "type": "double"
+    },
+    "target_index_suffix": {
+        "type": "text"
+    },
+    "aliases": {
+        "type": "object"
+    },
+    "force_unsafe": {
+        "type": "boolean"
+    }
+}
+```
+
+Parameter | Description | Type | Example | Required
+:--- | :--- |:--- |:--- |
+`num_new_shards` | The maximum number of primary shards in the shrunken index | integer | `5` | Yes, however cannot be used with `max_shard_size` or `percentage_of_source_shards`.
+`max_shard_size` | The maximum size in bytes of a shard on in the target index. | keyword | `5gb` | Yes, however cannot be used with `max_shard_size` or `percentage_of_source_shards`.
+`percentage_of_source_shards` | Percentage of the number of original primary shards. This indicates the minimum percentage by which the number of primary shards will shrink. Must be between 0.0 and 1.0 exclusive  | Percentage | `0.5` | Yes, however cannot be used with `max_shard_size` or `percentage_of_source_shards`.
+`target_index_suffix` | Suffix that will be appended to the target index to by shrunk | text | `shrunken` | No
+`aliases` | Aliases which will be added to the new index | object | `myalias` | No
+`force_unsafe` | If true, executes the shrink action even if there are no replicas | boolean | `false` | No
+
 ### close
 
 Closes the managed index.