Dry run auto migrate

[googollee]

• [x] Do only one thing
• [x] Non breaking API changes
• [x] Tested

What did this pull request do?

Add a flag Config.DryRunMigration to let AutoMigrate() return an error before changing the schema.

User Case Description

See https://github.com/go-gorm/gorm/issues/5635.

Notice

it requires https://github.com/go-gorm/gorm/pull/5644 (migrate check constraints without relationship constraints) and https://github.com/go-gorm/gorm/pull/5620 (postgres) to run tests succeed

[googollee]

Rebased with fix https://github.com/go-gorm/gorm/pull/5620.

[googollee]

Rebased with the latest commit.

[jinzhu]

In favor of https://github.com/go-gorm/gorm/pull/5689