test: add metadata and metadataKey test coverage

[Kubikusik]

Description:
Cover test coverage gap for setMetadata and setMetadataKey in
TokenUpdateTransactionUnitTests.cc to match the pattern of all other setters.

• Add mTestMetadataKey private member and getTestMetadataKey() accessor to test fixture
• Add GetSetMetadataFrozen test
• Add GetSetMetadataKey and GetSetMetadataKeyFrozen tests
• Extend ConstructTokenUpdateTransactionFromTransactionBodyProtobuf to set and
  verify metadata and metadata_key fields

Related issue(s):
Fixes #1235

Notes for reviewer:
Current implementation involves a helper function for setting body->set_allocated_metadata in the TokenUpdateTransactionBody protobuf, if there is a more intended solution I have missed, please do propose it as a change here.

Checklist

• Documented (Code comments, README, etc.)
• Tested (unit, integration, etc.)

[github-actions]

Hey @Kubikusik 👋 thanks for the PR!
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✅ Issue Link -- Linked to #1235 (assigned to you).

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[rwalworth]

Thanks for tackling this @Kubikusik! The test coverage additions are on the right track - the new GetSetMetadataFrozen, GetSetMetadataKey, and GetSetMetadataKeyFrozen tests and the protobuf deserialization assertions are exactly what #1235 asks for.

I left a few comments below. To answer your question from the PR description - there is a simpler way to set the metadata field on the protobuf body that avoids the helper function entirely. Details in the comment below!

[rwalworth]

Thanks for the updates @Kubikusik - all three items from my previous review are addressed. The bytesToProtobuf helper is gone, GetSetMetadataKey matches the non-frozen pattern, and the trailing newline is back. Running the workflows now and will merge once they pass!