ddl: set request source for cop requests that are used in adding index

[tangenta]

What problem does this PR solve?

Issue Number: close #41400

What is changed and how it works?

Set the request source for coprocessor requests that are used in adding index.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• wjhuang2016

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[Benjamin2037]

LGTM

[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 0ce2985

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-6.5: #41405.