SQL and Warehouse Materializations: support feature flags as advanced configuration options

[jonwihl]

The following feature flags should become permanent configuration options in the 'Advanced Configuration' section:

• allow_existing_tables_for_new_bindings
• retain_existing_data_on_backfill
• always_drop_tables_on_backfill

Support considerations:

• Should support migrate all existings feature flags to configuration options?

[jwhartley]

Internal thread