Support providing an index predicate for upserts. (#13822)

This is useful to upsert against a table which has a unique
partial index while avoiding conflicts.

synapse/storage/background_updates.py

@@ -533,6 +533,7 @@ class BackgroundUpdater:
             index_name: name of index to add
             table: table to add index to
             columns: columns/expressions to include in index
+            where_clause: A WHERE clause to specify a partial unique index.
             unique: true to make a UNIQUE index
             psql_only: true to only create this index on psql databases (useful
                 for virtual sqlite tables)