[Proposal] Support persistence of configuration items modified at runtime

[morningman]

Is your feature request related to a problem? Please describe.

Doris now support modified FE and BE config at runtime. But the modification will be invalid after process restart.
This will make the cluster operation and maintenance more complicated, because if the process restarts unexpectedly,
the configuration changes made during runtime will become invalid and need to be modified again.

Describe the solution you'd like

I'd like to support persistence of configuration items modified at runtime.
The modified configurations will be saved in fe_custom.conf/be_custom.conf under conf/ dir.

And when process restarts, it will first load config from fe.conf/be.conf, then load fe_custom.conf/be_custom.conf.
The config in xx_custom.conf will overwrite the same config item in fe.conf/be.conf

[yangzhg]

Nice job!