Making Credentials classes Serializable about google-auth-library-java

[garrettjonesgoogle]

One thing that's not clear to me from HttpServiceOptions is why it's necessary to have two layers of indirection, i.e. httpTransportFactoryClassName => httpTransportFactory => HttpTransport.

[mziccard]

HttpTransport implementations might have instance fields (see for instance NetHttpTransport.Builder). So while you could serialize them just using the class name and a no-arg constructor the deserialized transport could have have a different behavior (e.g. missing a proxy configuration).

We introduce an HttpTransportFactory and document that it should have a no-arg costructor so we can serialize it just using only the classname. To HttpTransportFactory we can add proper documentation and it's reasonable to assume/require that a factory has no instance fields and an empty constructor.

[garrettjonesgoogle]

Ok, that makes sense. This seems like a reasonable approach and I'm personally fine with using it here (google-auth-library-java).

[anthmgoogle]

I agree with adding this functionality to Credentials so that it can be used more broadly. There was no decision when this was originally created inconsistent with this.

[mziccard]

This was fixed in #69