Add abseil-cpp as a 3rd party library

[pjreiniger]

I wanted to upgrade protobuf as the more recent versions have better bazel support. The currently vendor'd version is from 9/22. Moving to a more recent release requires a dependency on abseil-cpp. I saw the #6607 and #6609, which also refer to abseil, albeit in a cmake case.

I'm happy to do the work, I just wanted approval for such a change before spending time on it.

[PeterJohnson]

Other than the fact I hate abseil with the flame of 1000 suns because it is the definition of dependency hell, if we have to upgrade, we have to upgrade. Can we at least build it all into a single library rather than the 50+ individual libraries it seems to have in distributions? Could we even bundle it all together with the protobuf thirdparty library (like how we do imgui)?