feat: allow reloading config with NOTIFY

Enables reloading the config by doing:
NOTIFY pgrst, 'reload config'

Adds an alias for reloading the schema cache:
NOTIFY pgrst, 'reload schema'

test/io-tests/configs/expected/no-defaults-with-db.config

@@ -5,7 +5,7 @@ db-extra-search-path = "public,extensions"
 db-max-rows = 1000
 db-pool = 1
 db-pool-timeout = 100
-db-pre-request = "custom_headers"
+db-pre-request = "test.custom_headers"
 db-prepared-statements = false
 db-root-spec = "root"
 db-schemas = "test,tenant1,tenant2"