Add TODO related to runtime dependency on Rake

It's a little bit awkward that we currently have to take a runtime dependency
on rake even though it is a build tool. This CL adds a TODO to explore moving
the Rakefile logic to extconf.rb so that we can drop the rake dependency.

PiperOrigin-RevId: 600936420

ruby/google-protobuf.gemspec

@@ -32,6 +32,10 @@ Gem::Specification.new do |s|
     s.add_development_dependency "rake-compiler-dock", "= 1.2.1"
   end
   s.required_ruby_version = '>= 2.7'
+  # TODO: evaluate removing Rakefile and moving logic to extconf.rb, so that we
+  # can remove this runtime dependency on rake. See the discussion here for
+  # more details:
+  # https://github.com/protocolbuffers/protobuf/pull/15203
   s.add_dependency "rake", ">= 13"
   s.add_development_dependency "ffi", "~>1"
   s.add_development_dependency "ffi-compiler", "~>1"