Online DDL: support DROP FOREIGN KEY statement

[shlomi-noach]

Description

For the ongoing, experimental foreign key support, this PR adds support for ALTER TABLE ... DROP FOREIGN KEY. As you might see from the change, there's very little to this PR; all the support has been created long ago, but due to an oversight we did not include DROP FOREIGN KEY in the logic.

Just to explain the change: in Online DDL we actually drop the foreign key on the shadow table. Since foreign key constraints, like check constraints, have schema-level naming scope, the shadow table's foreign key constraint is different than the original's.

Without this PR, a ALTER TABLE t DROP FOREIGN KEY t_id_fk fails because it attempts to drop the constraint t_id_fk from the shadow tables. But on the shadow table the constraint might be named e.g. fk_aulpn7bjeortljhguy86phdn9. The fix in this PR is to use the constraint mapping between original and shadow table.

Related Issue(s)

Tracking: #11975

Checklist

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