Add WebAPI overload that fetches protobuf instead of vdf

[xPaw]

I only changed the response, not the input.

The api accepts input_protobuf_encoded which means the overload could be changed to accept a proto request.

I updated ContentServerDirectoryService to use this (which by the way, could actually be executed as a service call over CMClient instead).

Thoughts?

[yaakov-h]

Overall I think it makes sense. Annoyingly that's one more set of generic type arguments to annotate for trimming, but that's the world we live in now.

[yaakov-h]

I keep coming back to this and having doubts about the name CallAsync<T> which (a) matches the existing CallAsync that returns KeyValue, and (b) isn't constrained and doesn't explain what T is.

We could rename it something like CallAsyncProtoBuf or something but I'm not particularly happy with any other name I've come up with.

Any suggestions, or should we just leave it as is for now?