fix: RenameIndex and DropIndex failing on CURRENT_SCHEMA()

[keif888]

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

What did this pull request do?

This pull request fixes issue #350, by retrieving the value of current_schema() when needed, and wrapping the result in a clause.Column within RenameIndex and DropIndex

PR #349 is another attempt to address this issue, in a slightly different way.
Which of these solutions is better I leave up to the Maintainers to decide.

User Case Description

As a developer I want to be able to drop indexes within a Postgres database using code as per the TestMigrateIndexes gorm testing function:

	if err := DB.Migrator().DropIndex(&IndexStruct{}, "Name"); err != nil {
		t.Fatalf("Failed to drop index for user's name, got err %v", err)
	}

As a developer I want to be able to rename indexes within a Postgres database using code as per the TestMigrateIndexes gorm testing function:

	if err := DB.Migrator().RenameIndex(&IndexStruct{}, "idx_index_structs_name", "idx_users_name_1"); err != nil {
		t.Fatalf("no error should happen when rename index, but got %v", err)
	}

Testing

This PR has been tested via the gorm test_all.sh script, with the go.mod file modified to use this version of the postgres driver, instead of the published version.
mod.go change:

replace gorm.io/gorm => ../

replace gorm.io/driver/postgres => ../../postgres

Test results:

GORM_DIALECT=postgres ./tests_all.sh
[+] up 1/1
 ✔ Container tests-postgres-1 Healthy                                                                               0.5s
testing postgres...
ok      gorm.io/gorm    1.024s
ok      gorm.io/gorm/callbacks  1.020s
ok      gorm.io/gorm/clause     1.682s
?       gorm.io/gorm/internal/lru       [no test files]
?       gorm.io/gorm/internal/stmt_store        [no test files]
ok      gorm.io/gorm/logger     1.014s
?       gorm.io/gorm/migrator   [no test files]
ok      gorm.io/gorm/schema     1.315s
ok      gorm.io/gorm/utils      1.018s
?       gorm.io/gorm/utils/tests        [no test files]
ok      gorm.io/gorm/tests      65.132s