feat(credential-providers): make credential providers aware of caller client region

[kuhe]

We previously introduced the concept of a "parentClientConfig" for credential providers that create another (inner) SDK client such as STS.

One problem was that if the provider was initialized outside of a client, this association between the client and credential provider function could not be made. This PR makes AWS credential providers accept a contextual client config, making them region aware at call time. The result is improved intuitiveness of default behavior, as shown below.

currently

⚠️ when using a CODE level credential configuration, any potential spawning of an inner client such as STS for AssumeRole must be passed the region in a redundant manner. This is because fromIni is a standalone function that can resolve credentials without the context of an SDK client.

import { DynamoDB } from "@aws-sdk/client-dynamodb";
import { fromIni } from "@aws-sdk/credential-providers";

const ddb = new DynamoDB({
  region: "eu-west-1",
  credentials: fromIni({
    clientConfig: { region: "eu-west-1" }, // required, but often forgotten
  }),
});

in this PR

Improved intuitiveness of credentials region resolution.

import { DynamoDB } from "@aws-sdk/client-dynamodb";
import { fromIni } from "@aws-sdk/credential-providers";

const ddb = new DynamoDB({
  region: "eu-west-1",
  // will default to the client's region instead of the commercial partition default (us-east-1)
  credentials: fromIni({}), 
});

// initialized outside a client
const credentialsProvider = fromIni();

// uses global default of us-east-1 as before, when invoked outside of a client.
const credentials = await credentialsProvider();

const ddb = new DynamoDB({
  region: "eu-west-1",
  credentials: credentialsProvider, // will default to the client's region if supplied to a client
});