Add support for IAMServiceAccountCredentials API

[owainhunt]

Adds support for the IAM Service Account Credentials API, specifically the signJwt endpoint.

Usage:

func signJWT() {
    let client = try IAMServiceAccountCredentialsClient(credentials: credentialsConfiguration,
                                           config: IAMServiceAccountCredentialsConfiguration,
                                           httpClient: client,
                                           eventLoop: myEventLoop)

    let payload: JWTPayload = MyPayload(name: "key", value: "value")

    client.api.signJWT(payload, serviceAccount: "[email protected]").map { response in
        print(response.signedJwt) // Prints JWT signed with the given service account's credentials 
    }
}

[owainhunt]

cc @nathanfallet @Andrewangeta

[owainhunt]

@nathanfallet can we get the tags mentioned in #53 set up please (or at least the 1.0) - I've got a corresponding PR ready to go for #54 that wraps this new functionality in vapor-community/google-cloud but it needs a tag to point to 🙂