feat: move common-protos to the monorepo

[bshaffer]

Move the package google/common-protos (https://github.com/googleapis/common-protos-php) over to the monorepo.
Renames the existing CommonProtos repo to CloudCommonProtos, and uses CommonProtos as the directory for the common protos for all APIs.

This has the following advantages:

1. Keeps things consistent with our other packages
2. Keeps everything in one place, so we don't forget about it
3. Keeps things consistent with our other packages
4. Will deploy documentation to cloud.google.com/php/docs/reference/
5. keeps things consistent with our other packages

NOTE: I have configured this to use the same repo (googleapis/common-protos-php), but the first push will require a --force, and will erase the history. I've tested this in composer, and upgrading to the new version / downgrading to the old one was not affected by rewriting the history, so this should not affect our users.

NOTE: This is definitely more of a chore than a feat, but I'd like to increment the version number to 0.5