feat: Exposes Google.Apis.Http.IHttpClientFactory in Rest.ClientBuilderBase

[amanda-tarafa]

See googleapis/google-api-dotnet-client#1864

After this has been merged and release, users can either reference the new Gax version directly or wait for an update Storage release. Then their code should look something like this:

// Users implementation of Google.Apis.Http.IHttpClientFactory.
var proxiedHttpClientFactory = new ProxyHttpClientFactory(proxy);

var scopes = new string[] { "https://www.googleapis.com/auth/devstorage.full_control" };
var serviceCredentialFromFile = (ServiceAccountCredential) GoogleCredential.FromFile(sa_file).UnderlyingCredential;
var initializer = new ServiceAccountCredential.Initializer(serviceCredentialFromFile.Id)
{
    ProjectId = serviceCredentialFromFile.ProjectId,
    Key = serviceCredentialFromFile.Key,
    User = serviceCredentialFromFile.User
    HttpClientFactory = proxiedHttpClientFactory,
    Scopes = scopes
};
var credential = GoogleCredential.FromServiceAccountCredential(new ServiceAccountCredential(initializer));

var clientBuilder = new StorageClientBuilder
{
    Credential = credential,
    // All HTTP client instances for API calls will be obtained from this factory
    HttpClientFactory = proxiedHttpClientFactory
}

var client = clientBuilder.Build();

Notice also that once googleapis/google-api-dotnet-client#1851 is in, setting a proxy for a credential will be much more simple as well.

[jskeet]

Nice! I'm impressed it's this simple :)

We may want to think about anything else we want to get into a new GAX release before releasing this... presumably as a workaround, users could create the StorageService themselves and then call the StorageClientImpl constructor, right?

[amanda-tarafa]

Yes, it's good that we really use the client factory for everything :).

And yes, users should be able to create the StorageService and then StorageClientImp themselves while we haven't released this.