Add cel-expr-python backend

[anuraaga]

This adds an implementation using cel-expr-python. It is much faster, orders of magnitude, but has many gotchas. This is currently a rough draft PR to show the concept and list up what it means

• We will need to keep the celpy fallback due to limited platform support for cel-expr-python. The engine glue code is large and duplicated by necessity
  • No musl wheels (probably will be fixed)
  • No free-threaded Python support (I haven't seen a Google project with support before)
  • Build requires Bazel so no sdist. Normally extensions like protobuf-py-ext can still be used with the relatively simple "install rust" prereq, but here it's impossible
• We can't add an automatic dependency on it using markers so it is always opt-in by user
  • No marker for musl vs glibc
  • No marker for free-threaded Python
• We have to copy protobuf-py messages to google.protobuf messages. There is some perf impact but we can consider it minimal. But our core engine becomes google.protobuf, even though we want our ecosystem to be built on protobuf-py