Expose generator and API for ProtoDescriptors and ProtoReflection

[FranzBusch]

In the discussion of #1762, @allevato brought up that the approach other languages take is generating ProtoDescriptors and exposing reflection APIs that use those descriptors to modify the value of fields.

[thomasvl]

This type of support has also come up in #1284, #1029, and #326.