Set up automatic copybara sync

[phisiart]

We shouldn't need to sync our internal codebase to this repo manually. Instead, a copybara service should automatically convert each internal change to a commit here.

[phisiart]

Update: We are making progress internally.

[phisiart]

Update: We have created an internal git repo that automatically syncs from our internal monorepo. After this gets stabilized, we will then sync the internal git repo to this repo.

[phisiart]

Update: Now we have this automatic sync flow: internal monorepo -> internal git repo -> github.

There are still further improvements to be made, but the current flow is already eliminating the need to manually sync to github.