Notable changes

C++17 is required to build the project
Fix a race condition in protobuf map accessor, occurring during concurrent access to a shared map (6dfb7c8)
Enforce recursion limits in parsing
Fix integer overflow on type conversions from double, uint values
Fix signed and unsigned integer overflow cases within C++ evaluator
Enable timestamp tests and add support for fixed timezones
Added support for bool keys to FieldBackedMapImpl
Update the has() macro support for C++ and fix enum resolution behavior when the qualified enum id is encoded within an identifier and not just within parse-only select expressions
Makes CEL parser tolerate trailing commas in struct constructors
Added support for CelType
Refactor protobuf wrapper to CelProtoWrapper
Removed gRPC as a build requirement, conformance is validated using raw pipes.

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