perf(config): read zone conf once

[qzhuyan]

Fixes EMQX-9801

Summary

Prior to this change, the default zone could be referenced by the listeners and it is not necessarily an item in the CONF PT if it is not defined under root zones by user .
Reading config for the zone must be done through the emqx_config:get_zone_conf/2 in which it first reads if the default zone exists in the config, otherwise it fallback to reading from the root of EMQX as the default fallback. If the default zone does exist but the field is undefined, it will fallback to read from default as well.

Generally, it has the following issues:

1. Inconsistent return val of emqx_config:get_zone_conf(default,[]) and emqx_config:get([zones, default]) that one of them return some value another read failure.

2. Always read the config twice if no user-defined default zone or the field is undefined. (first read miss then fallback to default) which hurts overall performance if read is done in the hot code path.

3. Read a zone with a none existing zone name (not default) will also return values that may confuse the users.

In this PR:

1. In CONF, The default zone which is referenced by the listeners is now explicitly inserted into the runtime config during emqx config initialization during emqx schema loading. the values are merged with zone global defaults and user-defined values.

2. in RAWCONF, it remains unchanged and it keeps a copy of user-defined configs for reference

3. When there is an update on the global defaults, it will be merged into the zones.

4. When there is an update on the existing zone, it is just a regular update no merge with global defaults.

5. When there is an insertion of new zones, they will be merged with zone global defaults

6. During initialization, loading config order is enforced. load zones after the other ROOTs.

Breaking changes:

1. Read the none-existing zone will fail now.
2. After emqx_schema is loaded, CONF PT stores the runtime config that has a default zone with complete children fields all set
3. CONF is now the runtime config for hot code path reading while RAWCONF has user-defined values and they are not in sync for root zones (<<"zones">>)

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/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

Checklist for CI (.github/workflows) changes

• If changed package build workflow, pass this action (manual trigger)
• Change log has been added to changes/ dir for user-facing artifacts update

[qzhuyan]

note, with many different attempts in the PR, need to squash the commits before the merge.

[qzhuyan]

also improve: emqx_mgmt_caps:get_caps

[qzhuyan]

need squash after review

[zhongwencool]

LGTM

[yanzhiemq]

Enhancements

• Reduced the overhead during configuration reads by optimizing the configuration read mechanism.

[yanzhiemq]

增强

• 通过优化配置读取机制减少读取配置的开销。