Add option callbacks to aws.Config for modifying default credentials

[rittneje]

Is your feature request related to a problem? Please describe.

We load our credentials via the default chain. In particular we are ultimately using stscreds.WebIdentityRoleProvider. We would like to be able to set the ExpiryWindow. That way we can generate presigned URLs that will be valid for 15 minutes, without worrying about whether the backing role credentials will expire sooner than that. However, right now this cannot be done, because we only get a credentials.Credentials instance, and the provider is a private field.

Describe the solution you'd like

Add options functions to the aws.Config to allow modifying the default credentials.

type Config struct {
    ...
    DefaultCredentialsOptions *DefaultCredentialsOptions
}

type DefaultCredentialsOptions struct {
    ...
    WebIdentityRoleProviderOption func(*stscreds.WebIdentityRoleProvider)
}

When the default credentials are being created, the corresponding function would be applied to the provider before it gets wrapped into a credentials.Credentials. If the function is nil then it is a no-op. In our case we would do this:

credOptions := &aws.DefaultCredentialsOptions{
    WebIdentityRoleProviderOption: func(creds *stscreds.WebIdentityRoleProvider) {
        creds.ExpiryWindow = 15 * time.Minute
    },
}

Describe alternatives you've considered
We can of course manually create the stscreds.WebIdentityRoleProvider ourselves but that is annoying and requires special code for anything running in EKS as opposed to a lambda.

[jasdel]

Thanks for taking the time to create this issue @rittneje. I've updated the PR with feedback on the design and proposal moving forward.

Also, this hasn't been merged yet, but I created a PR in the V2 SDK which addresses this issue directly without needing to use factories, since the v2 SDK already exposes many functional options for its components. aws/aws-sdk-go-v2#1523

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