[FEATURE] Default getCredentials for AWS Sigv4 Signer

[dblock]

Is your feature request related to a problem?

In #279 we introduced Sigv4 signing. To use this one must decide which version of AWS SDK to import, then implement different ways to get credentials depending on the version.

What solution would you like?

A version-agonostic default implementation. I want to be able to write the following regardless of the version of AWS SDK I am importing and without having to include any boilerplate for getting/refreshing credentials. And I want it to "just work" (TM) when I set, for example, credentials via ENV AWS_ variables.

const client = new Client({
  ...AwsSigv4Signer({
      region: 'us-east-1',
      node: "https://search-xxx.region.es.amazonaws.com",
  })
});

(this is similar to how one can use a client in the Java SDK).

[Kyeong1024]

you guys make @opensearch-project/opensearch/aws theseday. But when I install module like this npm i @opensearch-project/opensearch.
I CANNOT find aws folder at node_modules. Please fix this issue.
Thank you.

[wbeckler]

@Kyeong1024 Can you start a new issue with your request? (https://github.com/opensearch-project/opensearch-js/issues/new/choose)

[dblock]

I believe that wasn't released yet, see #303