Adds 'additional_settings' option to configure the underlying AWS client

[andsel]

Release notes

Add additional_settings option to fine-grain configuration of AWS client

What does this PR do?

Introduce the option additional_settings which is a map that, once the keys are symbolized and boolean strings becomes boolean values, is passed down to the creation of AWS SQS client.

Why is it important/What is the impact to the user?

This option permit the user to fine-grain configuring the client, for example to enable debugging logs at wire level, asin:

additional_settings => {
    http_wire_trace => true
}

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [x] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [x] Build the gem and try a run against SQS

How to test this PR locally

• build the gem gem build and install on a local Logstash instance
• grab account access key and token for an AWS account
• create an SQS

• setup an input like:

  input {
  sqs {
  queue => "<your queue name>"
  region => "<aws_region>"
  endpoint => "https://<aws_region>.amazonaws.com/<nnn>/<your queue name>"
  access_key_id => "aws_key"
  secret_access_key => "secret 1"
  role_arn => "arn:aws:iam::<nnn>:role/ElasticDeveloper"
  session_token => "secret 2" 
  
  additional_settings => {
    http_wire_trace => true
  }
  }
  }  

Related issues

• Closes: #59