Add option to specify a custom STS endpoint

Describe the feature

Add an option to specify the STS endpoint URL explicitly, as an alternative to deriving it automatically from the AWS region.

Use Case

I want to be able to use GitHub tokens for federated authentication to a Minio storage server - this uses an S3-compatible API for storage operations, but also includes an STS-compatible endpoint providing the AssumeRoleWithWebIdentity action to generate access credentials for the S3-compatible API using a federated identity token. As far as I can see this would be possible using the configure-aws-credentials action if there were the option to specify an explicit endpoint parameter in place of (or as well as) the region in CredentialsClient.stsClient():

configure-aws-credentials/src/CredentialsClient.ts

Lines 33 to 37 in fff2104

    
           this._stsClient = new STSClient({ 
        
             region: this.region, 
        
             customUserAgent: USER_AGENT, 
        
             requestHandler: this.requestHandler ? this.requestHandler : undefined, 
        
           });

Proposed Solution

add a new input sts-endpoint to the action
pass this through to the CredentialsClient constructor
if an endpoint has been specified, forward this to the STSClient constructor. Otherwise behave as normal, letting STSClient build its own default endpoint from the region name.

Other Information

No response

Acknowledgements

I may be able to implement this feature request
This feature might incur a breaking change

Thanks for the feature request, I'm not sure if we would consider this but would like to get more input from the community. Also I think this somewhat overlaps with #305.

	this._stsClient = new STSClient({
	region: this.region,
	customUserAgent: USER_AGENT,
	requestHandler: this.requestHandler ? this.requestHandler : undefined,
	});