refactor: distinguish between Unique and UniqueIndex

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• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

See go-gorm/gorm#6381.

based on go-gorm/gorm#6386

User Case Description

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I have a problem, When an index or constraint depends on the column,
this column cannot exec ALTER. See https://stackoverflow.com/questions/19460912/the-object-df-is-dependent-on-column-changing-int-to-double/19461205#19461205

Our driver doesn't seem to think about it. What should I do? @a631807682

[a631807682]

TestMigrateWithUniqueIndexAndUnique seems failed in sqlserver?

Sorry, I didn't see the comment, I'll check it out

[a631807682]

I have a problem, When an index or constraint depends on the column, this column cannot exec ALTER. See stackoverflow.com/questions/19460912/the-object-df-is-dependent-on-column-changing-int-to-double/19461205#19461205

Our driver doesn't seem to think about it. What should I do?

In this case, we need to modify the constraint or index first, but since we didn't support it before, we don't need to support it in this PR (may need to create another PR to fix it)

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It still has some problems which I'm fixing

After testing, it works well with go-gorm/gorm#6822