[feature](nereids) Add SHOW FRONTEND MASTER command and API

[heguanhui]

Implement dual access methods for master frontend node information:

• SHOW FRONTEND MASTER SQL command for query interface
• GET /api/frontend/master HTTP API for programmatic access
• Shared utility method for consistent data formatting
• Complete test coverage for both interfaces

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #xxx

Problem Summary:

Release note

None

Check List (For Author)

• Test

  • [ ] Regression test
  • [ ] Unit Test
  • [ ] Manual test (add detailed scripts or steps below)

• Behavior changed:

  • [ ] No.

• Does this need documentation?

  • [ ] No.

Check List (For Reviewer who merge this PR)

• [ ] Confirm the release note
• [ ] Confirm test cases
• [ ] Confirm document
• [ ] Add branch pick label

[Thearas]

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1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
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4. Which code was refactored and why was this part of the code refactored?
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