Initial implementation of Google API protobufs

[software-artificer]

This is a very initial proposal to address #876. I have compiled the latest main branch of the Google's API Common Protobuf repository, as it contains all the expected Protobufs referenced in the official docs. I would love to get feedback on this. Main points I am interested in:

1. Are we okay with a separate crate like this? Should this be part of the prost-types instead under a feature flag?
2. If separate crate is okay, how do we feel about crate name?
3. I have used api-common-protos repository as it is smaller and self-contained. The latest docs point to https://github.com/googleapis/googleapis/, which is much bigger/bloated. I don't feel like we need this, but happy to change over.
4. Is providing a crate like this sufficient, or should I make it an optional dependency of prost-build and/or prost and have some sort of .with_googleapis(true) flag that also injects includes properly and sets google:: namespace as a separate source obtained from this crate?
5. I am assuming here that we want to ship pre-generated code, but I am happy to figure out ways to make this part of the build.rs with selective generation of only included protos, if people feel strongly about it.

Any other thoughts or ideas are appreciated!

Fixes #876