* [fix](config) Fix uninitialized config validator
If we don't set any validator, the original implementation will cause
seg-fault due to nullptr of `RegisterConfValidator::_s_field_validator`,
which initial value relies on the first config validator definition.
We need to skip finding validator from
uninitialized `RegisterConfValidator::_s_field_validator`.
Encapsulate some http interfaces for better management and maintenance of doris clusters.
The http interface includes getting cluster connection information, node information, node configuration information, batch modifying node configuration, and getting query profile.
For details, please refer to the document:
`docs/zh-CN/administrator-guide/http-actions/fe/manager/`
Some invalid config value may cause BE work in an unexpected behavior,
this patch aim to support config validating when BE bootstrap and update BE's config by API
to reject invalid value.
This is a work to accomplish PR #4423
Support persistence of configuration items modified at runtime via HTTP API.
```
FE:
GET /api/_set_config?key=value&persist=true
BE
POST /api/update_config?key=value&persist=true
```
The modified config will be saved in `fe_custom.conf` or `be_custom.conf`.
And when process starts, it will load `fe.conf/be.conf` first, then `fe_custom.conf/be_custom.conf`.
In the past, when we want to modify some BE configs, we have to modify be.conf and then restart BE.
This patch provides a way to modify configs in the type of 'threshold', 'interval', 'enable flag'
when BE is running without restarting it.
You can update a single config once by BE's http API: `be_host:be_http_port/api/update_config?config_name=new_value`
