Add IAM authenticator

[gl-johnson]

Rough POC for adding IAM authentication to the Go SDK.

NOTE: This depends on the AWS config existing on the system, which should be the case for any user of the AWS CLI or when running on ec2 or lambda.

Usage example:

	config := conjurapi.Config{
		ApplianceURL: "http://localhost:3000",
		Account:      "cucumber",
		AuthnType:    "iam",
		ServiceID:    "aws",
		HostID:       "conjur/authn-iam/aws/production/578847545830/glen",
	}

	client, err := conjurapi.NewClientFromEnvironment(config)
	if err != nil {
		fmt.Println("Error creating Conjur client:", err)
		return
	}

	resp, err := client.WhoAmI()

	fmt.Println(string(resp))

Improvements:

• Add tests
• If configured host ID doesn't match the AWS config, we could attempt to assume the AWS role and fetch temporary credentials like so:

	svc := sts.NewFromConfig(cfg)
	credsProvider := stscreds.NewAssumeRoleProvider(svc, "arn:aws:iam::578847545830:role/MyAppRole")
	credentials, err := credsProvider.Retrieve(ctx)
	if err != nil {
		fmt.Println("Error retrieving credentials:", err)
		return nil
	}

	return &credentials

[codeclimate]

Method Client.IAMAuthenticateHeaders has 6 return statements (exceeds 4 allowed).

[codeclimate]

Code Climate has analyzed commit 74d41ee and detected 2 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	2

The test coverage on the diff in this pull request is 4.5% (50% is the threshold).

This pull request will bring the total coverage in the repository to 81.4% (-5.0% change).

View more on Code Climate.