[P4Testgen] Basic support for @p4runtime_translation and @p4runtime_translation_mappings.

[fruffy-g]

This PR adds support for @p4runtime_translation and @p4runtime_translation_mappings in the Protobuf IR back end of P4Testgen. There are two phases.

1. Since the P4Runtime translation annotations are only attached to the basic type definitions, we add a pass which propagates the annotations to any control plane element which uses the type. We can now reference the annotations when generating tests.

2. In the Protobuf IR back end, we add two checks for @p4runtime_translation and @p4runtime_translation_mappings. For @p4runtime_translation, we simply override the behavior of the getFormatOfNode function. If a @p4runtime_translation annotation is present, we return the value mapped in the annotation. Otherwise we default for normal behavior.

3. @p4runtime_translation_mappings are experimental and not well defined, so we simply brute-force inference here. We generate a <cstring, cstring> map which maps the second element in the pair to the first. We also parse the annotation as a list expression. We add a wrapper to formatNetworkValue which overrides the returned value if it is contained in the map.

4. Tests are missing. These properties are difficult to test with the current infrastructure. We could try to parse some programs which have these annotations.

[smolkaj]

The name of the translated type?