cmek split 2/8: TiKV encryption metadata HTTP client (#4571)

[ti-chi-bot]

This is an automated cherry-pick of #4571

What problem does this PR solve?

Issue Number: ref #3943

This is split PR 2/8 from #3955.

What is changed and how it works?

This PR adds TiKV-side encryption metadata retrieval:

• Add TiKV encryption metadata HTTP client.
• Add JSON/bytes decoding helpers for metadata payloads.
• Parse and normalize metadata for keyspace-level encryption usage.
• Add unit tests for endpoint behavior and decoding logic.

This PR focuses only on metadata retrieval and parsing, and does not yet wire server lifecycle.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Questions

Will it cause performance regression or break compatibility?

No direct user-facing behavior change in this split; it introduces a client used by later PRs.

Do you need to update user documentation, design documentation or monitoring documentation?

No.

Release note

None

Summary by CodeRabbit

• New Features

  • Added encryption metadata client for TiKV store integration with support for protobuf encoding and HTTP communication with configurable TLS.

• Tests

  • Added comprehensive test coverage for encryption metadata retrieval, including edge cases, error handling, and protocol validation.

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenfyzhong

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