Added credential provider example

[siennathesane]

Added a credential provider example for the AWS for Go SDK v2. There is a matching PR with the complete code at localstack/localstack-aws-sdk-examples#9.

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[gregfurman]

Hey. Thanks for this! I'm always a fan of Golang + LocalStack getting more love 😄

Couple of remarks on this. First, the CI is failing becase of linting issues (see failed job here). Also maybe give the best practices a lookthrough which could help diagnose/fix this.

In addition, I think for the purposes of documentation, it would probably be a better idea to provide users' with a simpler minimum-viable-solution. While what this PR proposes would obviously work, I'm concerned perhaps the scope is a bit large.

What about something like this?

const (
	AwsLocalEndpoint        = "http://localhost:4566"
	AwsLocalCredentialsName = "AwsLocalCredentials"
	AwsLocalDefaultRegion   = "us-east-1"
	AwsLocalAccountId       = "000000000000"
	AwsLocalAccessKey       = "test"
	AwsLocalSecret          = "test"
)

func main() {
	// build an aws.Credential-compliant configuration
	localstackCredentials := aws.Credentials{
		AccessKeyID:     AwsLocalAccessKey,
		SecretAccessKey: AwsLocalSecret,
		SessionToken:    "SESSION",
		Source:          "LocalStack hardcoded credentials.",
	}

	awsCfg, err := config.LoadDefaultConfig(context.TODO(),
		config.WithCredentialsProvider(credentials.StaticCredentialsProvider{
			Value: localstackCredentials,
		}),
		config.WithDefaultRegion(AwsLocalDefaultRegion),
		// Optionally, add more LoadOptions...
	)

	// ...

}

It could probably be slimmed down even further to just focus on the credential provider.

What are your thoughts?

[siennathesane]

The matching PR in the examples repo contains all the necessary code, so I don't think this update is required. I included it since it's a common use case, but I don't feel strongly about this PR. Feel free to update or close it, I'm fine with either