Vendor Abseil CPP for Consistency

[iwillspeak]

Rather than downloading and buidling this each time in the CI we should proabbly just add Abseil into the thirdparty folder, and then include it in our CMake project as a direct forlder reference as we do for the main RE2.

[iwillspeak]

The advantage here is we could then pin to a specific Abseil version, and more easily ensure we statically link to libabsl.