[FEATURE] Alllow overriding the aws service identifier in AwsSigv4Signer

[bwg]

Is your feature request related to a problem?

Currently the service identifier is hard coded to es with no way to override.

https://github.com/opensearch-project/opensearch-js/blob/2.1/lib/aws/AwsSigv4Signer.js#L73

What solution would you like?

Allow overriding the default with a user provided value from opts

request.service = opts.service || 'es';

What alternatives have you considered?

rolling our own signing functionality

[wbeckler]

This makes sense. Feel free to contribute a PR.

[dblock]

Closing via #333