Adding sample configuration for auth_type and aws_iam

Signed-off-by: Naveen Tatikonda <navtat@amazon.com>

_clients/logstash/ship-to-opensearch.md

@@ -75,3 +75,72 @@ output {
     health | status | index | uuid | pri | rep | docs.count | docs.deleted | store.size | pri.store.size
     green | open | logstash-logs-2021.07.01 | iuh648LYSnmQrkGf70pplA | 1 | 1 | 1 | 0 | 10.3kb | 5.1kb
     ```
+
+## Adding different Authentication mechanisms in the Output plugin
+
+## auth_type to support different authentication mechanisms
+
+In addition to the existing authentication mechanisms, if we want to add new authentication then we will be adding them in the configuration by using auth_type
+
+Example Configuration for basic authentication:
+
+```yml
+output {    
+    opensearch {        
+          hosts  => ["https://hostname:port"]     
+          auth_type => {            
+              type => 'basic'           
+              user => 'admin'           
+              password => 'admin'           
+          }             
+          index => "logstash-logs-%{+YYYY.MM.dd}"       
+   }            
+}               
+```
+### Parameters inside auth_type
+
+- type (string) - We should specify the type of authentication
+- We should add credentials required for that authentication like 'user' and 'password' for 'basic' authentication
+- We should also add other parameters required for that authentication mechanism like we added 'region' for 'aws_iam' authentication
+
+## Configuration for AWS IAM Authentication
+
+To run the Logstash Output Opensearch plugin using aws_iam authentication, simply add a configuration following the below documentation.
+
+Example Configuration:
+
+```yml
+output {        
+   opensearch {     
+          hosts => ["https://hostname:port"]              
+          auth_type => {    
+              type => 'aws_iam'     
+              aws_access_key_id => 'ACCESS_KEY'     
+              aws_secret_access_key => 'SECRET_KEY'     
+              region => 'us-west-2'         
+          }         
+          index  => "logstash-logs-%{+YYYY.MM.dd}"      
+   }            
+}
+```
+
+### Required Parameters
+
+- hosts (array of string) - AmazonOpensearchService domain endpoint : port number
+- auth_type (Json object) - Which holds other parameters required for authentication
+    - type (string) - "aws_iam"
+    - aws_access_key_id (string) - AWS access key
+    - aws_secret_access_key (string) - AWS secret access key
+    - region (string, :default => "us-east-1") - region in which the domain is located
+    - if we want to pass other optional parameters like profile, session_token,etc. They needs to be added in auth_type
+- port (string) - AmazonOpensearchService listens on port 443 for HTTPS
+- protocol (string) - The protocol used to connect to AmazonOpensearchService is 'https'
+
+### Optional Parameters
+- The credential resolution logic can be described as follows:
+    - User passed aws_access_key_id and aws_secret_access_key in configuration
+    - Environment variables - AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY (RECOMMENDED since they are recognized by all the AWS SDKs and CLI except for .NET), or AWS_ACCESS_KEY and AWS_SECRET_KEY (only recognized by Java SDK)
+    - Credential profiles file at the default location (~/.aws/credentials) shared by all AWS SDKs and the AWS CLI
+    - Instance profile credentials delivered through the Amazon EC2 metadata service
+- template (path) - You can set the path to your own template here, if you so desire. If not set, the included template will be used.
+- template_name (string, default => "logstash") - defines how the template is named inside Opensearch