NI-RFSG gRPC enum metadata cleanup

[batzuu]

• [x] This contribution adheres to CONTRIBUTING.md.
• [ ] ~~I've updated CHANGELOG.md if applicable.~~
• [ ] ~~I've added tests applicable for this pull request~~

What does this Pull Request accomplish?

Updates NI-RFSG function metadata to prepare for upcoming gRPC support:

• Removes grpc_enum (eg: NiRFSGInt32AttributeValues), grpc_mapped_enum fields for parameters which are not enums. (These were added as placeholders in the first RFSG metadata addition PR.)
• Adds grpc_enum fields for few string enums parameters required by grpc-device server.

List issues fixed by this Pull Request below, if any.

None

What testing has been done?

gRPC related fields in RFSG metadata are not consumed in the builds yet. These changes are prerequisite to allow us to enable gRPC codegen in upcoming PRs.