[Draft] Add generated client files to fcm

[maceonthompson]

[Draft] The result of moving autogenerated fcm client code using GAPIC into the messaging directory. Subsequent PRs will be made into this branch as development continues (namely, migrating to the generated client over messaging-api-request-internal.ts

This is part of an ongoing internship project/experiment in the feasibility of using autogenerated clients, which will hopefully make supporting multiple apis across multiple languages easier in the long term.

[hiranya911]

I see that you've nested generated within the existing src directory, which is not quite what I had in mind. I was suggesting that we put generated at the root of the repo (same level as src). However, this is likely to complicate our build quite a bit (TypeScript will have to pull sources from 2 directories, and combine the compiled output into one).

So lets use the structure you currently have for now. Later we can discuss other approaches for organizing these files in our repo.