Backport service/postgres changes from v0.131.0

Cherry-picks the postgres-only portion of the following auto-generated
"Update SDK API" PRs from main onto v0.127.0:

- #1644 (bb51d91): model.go additions
- #1649 (05bc320): impl.go + model.go (adds ReplaceExisting)
- #1651 (7aae318): api.go, impl.go, interface.go, model.go
- #1657 (a4ef2fa): model.go

Files replaced wholesale with their v0.131.0 (5275d1a) versions:
  service/postgres/{api,impl,interface,model}.go
  experimental/mocks/service/postgres/mock_postgres_interface.go
  experimental/mocks/service/postgres/mock_undelete_project_operation_interface.go (new)

Co-authored-by: Isaac
Signed-off-by: Tanmay Rustagi <tanmay.rustagi@databricks.com>

Author: tanmay-db