server/refactor: use `Set`/`HashSet` to deduplicate function names in `runTxWithMetadataCheck`

The root cause of [graphql-engine#8643: Migrations fail if duplicate functions exist](https://github.com/hasura/graphql-engine/issues/8643) is that the list of functions to consider for metadata diff checks ends up with duplicates when a function is tracked both as root field and as computed field:

https://github.com/hasura/graphql-engine/blob/f80ec1d543e0e3e4711fe72bb8f19c097e6897c1/server/src-lib/Hasura/Backends/Postgres/DDL/RunSQL.hs#L303

The proper fix is to ensure that we use [sets](https://hackage.haskell.org/package/containers-0.6.6/docs/Data-Set.html)/[hashsets](https://hackage.haskell.org/package/unordered-containers-0.2.19.1/docs/Data-HashSet.html) in all the functions involved instead of lists in order to statically ensure no duplicates appear by accident.

Start with server/src-lib/Hasura/Backends/Postgres/DDL/RunSQL.hs around the linked line and trace runTxWithMetadataCheck plus the functions that assemble its function list. Confirm the duplicate root-field/computed-field case from graphql-engine#8643, then verify that the affected paths use sets and that migrations with duplicate functions no longer fail.

Written by the indexing model from the issue text.