Add javadoc and convenience methods for Base64Binary

[lmsurpre]

Is your feature request related to a problem? Please describe.
I was trying to use Base64Binary.Builder and noticed that the Builder.value(String) method doesn't have javadoc.

Describe the solution you'd like
We need to add that so that we know whether to pass a string that has already been base64-encoded or if it will automatically base64-encoded string that was passed.

Also, I'd like there to be a Base64Binary.of(byte[]) method