[FEATURE]: Add a feaure so it supports on or where clause on indexes

[shashanksingh2002]

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

A feature where we can add where clause on indexes meaning I should be able to add a unique constraint on only those columns where is_archived_at column is not null.

Something like this

unique().on(
table.squadNo,
table.startTime,
table.courseSlug,
table.facilitator,
).where((table) => table.isArchivedAt !== null)