NI-RFSG gRPC enum metadata cleanup

[batzuu]

• 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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.63%. Comparing base (cf435c0) to head (00aad10).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2139   +/-   ##
=======================================
  Coverage   89.62%   89.63%           
=======================================
  Files          61       61           
  Lines       17308    17308           
=======================================
+ Hits        15513    15514    +1     
+ Misses       1795     1794    -1     

Flag	Coverage Δ
nidcpowersystemtests	94.65% <ø> (+0.04%)	⬆️
nidcpowerunittests	89.53% <ø> (ø)
nidigitalsystemtests	92.26% <ø> (ø)
nidigitalunittests	68.44% <ø> (ø)
nidmmsystemtests	92.72% <ø> (ø)
nifakeunittests	85.52% <ø> (ø)
nifgensystemtests	94.61% <ø> (ø)
nimodinstsystemtests	73.85% <ø> (ø)
nimodinstunittests	94.20% <ø> (ø)
nirfsgsystemtests	76.53% <ø> (ø)
niscopesystemtests	92.94% <ø> (ø)
niscopeunittests	43.20% <ø> (ø)
nisesystemtests	91.50% <ø> (ø)
niswitchsystemtests	82.03% <ø> (ø)
nitclksystemtests	94.87% <ø> (ø)
nitclkunittests	98.26% <ø> (ø)

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see 1 file with indirect coverage changes

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[batzuu]

Build failure because of flake8 F842 warning is expected till this PR which adds a noqa is merged and I pull in changes to this branch. I have raised this PR before the other PR's merge so that changes in this PR are available to be reviewed in the meantime.