Improvement/add regions option support

[aoutfast]

Description

This PR adresses the issue with aws4 package usage, when it tries to get the AWS Region from the hostname.

Use Case

When the AWS Elasticsearch domain is under a custom domain the signature won't be valid. If the node is hosted in another region than us-east-1

eg.

• Expected domain Resulting Signature Valid ✅
  https://xxxx.<region>.es.amazonaws.com

• Aliased Domain Resulting Signature Invalid ❌
  https://search.mydomain.com

Snippet responsible for the region in aws4

this.region = request.region || hostParts[1] || 'us-east-1'

Resolution

Adding signOpt as an optional param to generateAWSConnectionClass

declare type SignOpt = {
  service: 'es';
  region?: string;
}

function generateAWSConnectionClass(credentials: Credentials, signOpts?: SignOpt)