Add an admin API endpoint to support per-user feature flags (#15344)

changelog.d/15344.feature

docs/admin_api/experimental_features.md

synapse/_scripts/synapse_port_db.py

synapse/rest/admin/__init__.py

synapse/rest/admin/experimental_features.py

synapse/storage/databases/main/__init__.py

synapse/storage/databases/main/experimental_features.py

...chema/main/delta/76/03_per_user_experimental_features.sql

tests/rest/admin/test_admin.py