add conformance test suite

[anuraaga]

This continues from #251 to hopefully get into a shape that can be merged, and then iterated to add more features / fixes to pass more of the conformance test.

I have redesigned things quite a bit to address feedback and simplify (IMO) things

• Separate a generate.rs script for doing all preprocessing of cel-spec. build.rs only checks for version mismatch and fails the build if there is one
  • To address point of not regenerating on every build
• Avoid the complexity of git submodules by fetching the zip archive during generation.
• Use protox for proto compilation and prost-reflect for textproto parsing to avoid protoc dependency
• Use standard cargo test as much as possible by codegenning test files themselves from the textproto. This allows test reports to show just like any cargo unit tests, no need for us to implement reporting logic. And we don't depend on cel-spec at all during test execution, it's only needed during the generation step
• A system using a text file of ignored tests to reference during generation to mark failing tests as #[ignored]. In the future, let's make this file empty!

AI disclaimer: I used AI for autocomplete and also to generate much logic that is mostly just string crushing. I read every single line and cleaned up / rewrote significant parts - for better or worse, I stand by each line of code.

[alexsnaps]

Hey @anuraaga,
Thanks for taking on that work 🙏
From a quick look, what would you think if we'd "#[should_panic(expected = "This is known limitation")]" instead of #[ignore]'ing the known tests that fail? That way, when one actually passes, it would be called out explicitly. Wdyt?

[anuraaga]

Great idea @alexsnaps updated to that. I left out an error expectation since the message doesn't matter for the test, just whether it fails or not.

[alexsnaps]

Sorry, somewhat slow following up on this, but haven't lost track of this PR 🙏