Add test for API deprecation/removal

[AlekSi]

Talos is not 1.0 yet, so technically, we can break our APIs in incompatible ways anytime. In practice, we are not doing that because that would make everyone's life harder. Still, we want to clean up our APIs before the 1.0 release: use common field names and conventions, rework APIs that are hard to use correctly, etc.

The existing prototool break check is a part of the solution, but not all of it.

I propose to modify an existing apid_test (and maybe find a better place for it) to check deprecated options and comments in .proto files and fail if a deprecated entity is not removed on time.

For example:

message ListRequest {
  // Use recursion_depth instead.
  // This field will be removed in v0.16.
  bool recurse = 2 [deprecated = true];
  int32 recursion_depth = 3;

The test will fail if the current version is 0.16.