ddl: support read generated columns with copr for adding index

[tangenta]

What problem does this PR solve?

Issue Number: ref #35983

Problem Summary:

In #39191, I left a TODO about the support of reading the generated column with coprocessor.

What is changed and how it works?

This PR supports reading the generated columns with coprocessor during 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:

• Benjamin2037
• wjhuang2016

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[Benjamin2037]

LTGM

[Benjamin2037]

/merge

[ti-chi-bot]

@Benjamin2037: /merge is only allowed for the committers, you can assign this pull request to the committer in list by filling /assign @committer in the comment to help merge this pull request.

In response to this:

/merge

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[hawkingrei]

/merge

[ti-chi-bot]

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

Commit hash: 6bebf55

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! [2] CI still failing after this pr merged.
These failed integration tests don't seem to be introduced by the current PR.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/mybatis-test	🔴 failed 1, success 0, total 1	11 min	Existing failure
idc-jenkins-ci-tidb/integration-ddl-test	🔴 failed 1, success 5, total 6	5 min 13 sec	Existing failure
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	32 min	Existing passed
idc-jenkins-ci/integration-cdc-test	🟢 all 39 tests passed	20 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	13 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	12 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 54 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 34 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 44 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed