ddl: Support the operation of dropping multi-indexes in one statement

[ou-bing]

What problem does this PR solve?

Issue Number: close #14765

Problem Summary:

What is changed and how it works?

Proposal: xxx

What's Changed:

How it Works:

ALTER TABLE tbl_name
    [alter_specification [, alter_specification] ...]

alter_specification:
| DROP [INDEX] index_name

Related changes

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

Check List

Tests

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

Side effects

• Performance regression
  • Consumes more CPU
  • Consumes more MEM
• Breaking backward compatibility

Release note

• DDL: Support the operation of dropping multi-indexes in one statement