Document canonical CEL extensions with stack availability

[TristonianJones]

The CEL team supports a number of canonical extension packages which enable features
beyond the core CEL specification as opt-in packages. Historically, cel-spec has documented
proposals for new extensions via wiki, but this never really invited additional feedback from
the community and made extension discovery and availability difficult to discover.

Proposal:

• Introduce extensions with versioning information in the cel-spec docs
• Provide information about availability within the Java, Go, C++, C, and Python stacks
• Revamp the proposal standard where approval includes a set of conformance tests in cel-spec