added: cli option --dump-config prints loaded config and exits

This is most useful for automated tests for upcoming configuration
features. Can also be used for debugging.

test/io-tests/configs/expected/defaults.config

@@ -0,0 +1,24 @@
+db-uri = "required"
+db-schema = "required"
+db-anon-role = "required"
+db-pool = 10
+db-pool-timeout = 10
+db-extra-search-path = "public"
+db-channel = "pgrst"
+db-channel-enabled = false
+db-tx-end = "commit"
+db-prepared-statements = true
+server-host = "!4"
+server-port = 3000
+server-unix-socket = ""
+server-unix-socket-mode = "660"
+openapi-server-proxy-uri = ""
+jwt-secret = ""
+jwt-aud = ""
+secret-is-base64 = false
+role-claim-key = ".\"role\""
+max-rows = ""
+pre-request = ""
+root-spec = ""
+raw-media-types = ""
+log-level = "error"