fix: Config API can't get the default values from raw config

[zhongwencool]

Fixes 9210

If we define mqtt in emqx.conf as:

mqtt {
  max_qos_allowed = 1 
}

then GET /configs/global_zones will return ...mqtt {max_qos_allowed: 1}..., the mqtt's other field is missing.

This PR tries to get the disappearing defaults back.

mqtt {
  await_rel_timeout = "300s"
  exclusive_subscription = false
  idle_timeout = "15s"
  ignore_loop_deliver = false
  keepalive_backoff = 0.75
  max_awaiting_rel = 100
  max_clientid_len = 65535
  max_inflight = 32
  max_mqueue_len = 2000
  max_packet_size = "11MB"
  max_qos_allowed = 1
  max_subscriptions = "infinity"
  max_topic_alias = 65535
  max_topic_levels = 65535
  mqueue_default_priority = "lowest"
  mqueue_priorities = "disabled"
  "mqueue_store_qos0" = true
  peer_cert_as_clientid = "disabled"
  peer_cert_as_username = "disabled"
  response_information = ""
  retain_available = true
  retry_interval = "30s"
  server_keepalive = "disabled"
  session_expiry_interval = "2h"
  shared_subscription = true
  strict_mode = false
  upgrade_qos = false
  use_username_as_clientid = false
  wildcard_subscription = true
}

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/ dir
• In case of non-backward compatible changes, reviewer should check this item as a write-off, and add details in Backward Compatibility section

Backward Compatibility

More information

[coveralls]

Pull Request Test Coverage Report for Build 3494987645

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 37 unchanged lines in 9 files lost coverage.
• Overall coverage decreased (-0.04%) to 77.966%

---

Files with Coverage Reduction	New Missed Lines	%
apps/emqx/src/emqx_config.erl	1	84.83%
apps/emqx/src/emqx_channel.erl	1	87.37%
apps/emqx_conf/src/emqx_conf_schema.erl	1	92.31%
apps/emqx/src/emqx_cm.erl	2	90.37%
apps/emqx/src/emqx_flapping.erl	3	80.85%
apps/emqx_gateway/src/emqx_gateway_cm.erl	3	73.45%
apps/emqx/src/emqx_broker.erl	4	82.97%
apps/emqx_management/src/emqx_mgmt_cli.erl	5	27.43%
apps/emqx/src/emqx_reason_codes.erl	17	86.76%

Totals
Change from base Build 3494892781:	-0.04%
Covered Lines:	20888
Relevant Lines:	26791

---

💛 - Coveralls