Support get_config & Optimize handling http request

[solotzg]

What problem does this PR solve?

Issue Number: close #1933
pingcap/tidb#28838

What is changed and how it works?

What's Changed:

• enhance interface fn_handle_http_request & add interface fn_get_config tidb-engine-ext#34
• add interfaces to get config info from tiflash by http://{status-addr}/config
  • got {"raftstore-proxy":xxxx,"engine-store":xxx}
• enhance interface about handling http request

Related changes

• PR to update pingcap/docs/pingcap/docs-cn:
• Need to cherry-pick to the release branch:

Check List

Tests

• Integration test

• Manual test (add detailed scripts or steps below)

  • modify https://github.com/ichn-hu/tidb/blob/master/executor/memtable_reader.go#L197-L201 to enable tiflash
  • run sql select * from INFORMATION_SCHEMA.CLUSTER_CONFIG or sql show config can get config info about tiflash

Side effects

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• birdstorm

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[solotzg]

/run-all-tests

[birdstorm]

LGTM

[solotzg]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: 26e249d