Add support for grpc-netty

[dsyer]

• I have properly formatted metadata files (see CONTRIBUTING document)
• I have added thorough tests. (see this)

[amitbansal26]

When this will be merged ?

[gradinac]

Hey @dsyer! :) Sorry for a late answer on netty/netty#13039 (comment) - I'll continue the posts here as it's directly related to this PR.
I don't think we currently have a way to use existing metadata from other libraries in tests for a particular library. In this case, it would be best if we could use the existing metadata, but I think we can have the required netty metadata manually specified for now.
Regarding netty/netty#13039 (comment), that's a very good question. It would not make the reflection registration redundant - if a type X is reachable, we do not register anything from type X for reflection by default. In this way, we tell native-image the following: if this type does end up in the image, also register the following for it.

[dsyer]

So how do we get this merged? We need another approval (from whom)? Or something else?

[dnestoro]

@dsyer sorry for the delay, we were on holidays so we didn't merge anything these days.