This PR implements lookup of service descriptor and method descriptor objects in Ruby as described in issue https://github.com/protocolbuffers/protobuf/issues/14891.
It contains three implementations - one for the CRuby extension API, one for JRuby, and one for FFI.
With this patch,
* `DescriptorPool#lookup('fully.qualified.service.name')` works and returns a `Google::Protobuf::ServiceDescriptor` object
* You can call `#options` on that to get the service options
* You can call `#methods` on that to get the services' methods as `Google::Protobuf::MethodDescriptor` objects,
* You can call `MethodDescriptor#options` to get method options
* You can also get the streaming flags & input/output types of the method with `#input_type`, `#output_type`, `#client_streaming`, and `#server_streaming`.
In order to make the FFI implementation work, I had to mark some more methods in the UPB header as exported - I guess that's something which will have to be done on the UPB side, like this 01fed1cc1b
CC @dazuma & @haberman from the original issue, and @JasonLunn (since you work on protobuf it seems - small world!)
I apologies for the large volume of copy-pasta'd code from the existing descriptor class implementations into the new ones - I felt this was probably better than designing new abstractions to reduce it off the bat though; this feels like it "fits in" with the existing implementation.
Closes#15817
KJ Tsanaktsidis