Convert libprotobuf to Bazel

Since this is a one-off just for building libprotobuf that will be removed when we handle cargo output more generically (b/297364081), I didn't write a unit test for this CL.

Test: b build //external/rust/crates/protobuf:libprotobuf
Bug: 295925256
Change-Id: I00cf44d54be27a09c184a96c13b250a2e54e2d10