reflection: add a proto option to let servers exclude selected services/methods/fields

[kindermoumoute]

Problem

Reflection is all-or-nothing: once reflection.Register runs, every descriptor is exposed. Projects often need to show only stable RPCs while iterating on new ones.

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Proposal

Add a visibility option (similar to google.api.visibility) that reflection implementations must respect.

message VisibilityOptions {
  enum Level {
    PUBLIC   = 0; // default – expose
    INTERNAL = 1; // hide from reflection
  }
  Level level = 1;
}

Anything tagged INTERNAL is omitted from reflection responses.

Any thoughts?