Resolve warning for missing assign in GRPCWithOptions plugin

[aj-foster]

Hi there 👋🏼

When using the GRPCWithOptions plugin, a warning is emitted (twice):

warning: assign @module_doc? not available in EEx template. Please ensure all assigns are given as options. Available assigns: [:module, :service_name, :methods, :descriptor_fun_body, :version]
  (eex 1.17.3) lib/eex/engine.ex:140: EEx.Engine.fetch_assign!/2
  (elixir 1.17.3) src/elixir.erl:386: :elixir.eval_external_handler/3
  (stdlib 6.1.2) erl_eval.erl:904: :erl_eval.do_apply/7
  (stdlib 6.1.2) erl_eval.erl:479: :erl_eval.expr/6
  (stdlib 6.1.2) erl_eval.erl:648: :erl_eval.expr/6
  (stdlib 6.1.2) erl_eval.erl:271: :erl_eval.exprs/6

This PR resolves the issue by creating a @module_doc? assign similar to the one available in the GRPC plugin.

❤️

[drowzy]

Thanks 😄!