Replace protobuf/abseil submodules with vendored sources

[kateinoigakukun]

Fixes #2000.

SwiftPM recursively checks out git submodules during package resolution. The protobuf and abseil submodules are large C++ repositories, but every consumer has to pay the cost of fetching them for every checkout because SwiftPM cannot cache submodules as their bare repositories.

This PR replace protobuf/abseil submodules with vendored sources.

Commits

1. Add protobuf vendor update automation
2. Remove protobuf and abseil submodule
3. Vendor protobuf and abseil

Reviewers can focus on the first commit; commits 2 and 3 are mechanical.

Alternative Considered

git subtree was also considered but ruled out: it does not support selective path filtering across multiple directories (we only need a subset of files from each repo).