Add: user define config

[A-Wanderer]

What this PR does:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

add user define config so that user can define somthing in config file

[codecov-commenter]

Codecov Report

Merging #1640 (1eb4eb9) into 3.0 (a2c0731) will increase coverage by 0.29%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##              3.0    #1640      +/-   ##
==========================================
+ Coverage   41.05%   41.35%   +0.29%     
==========================================
  Files         253      254       +1     
  Lines       14501    14541      +40     
==========================================
+ Hits         5953     6013      +60     
+ Misses       7859     7835      -24     
- Partials      689      693       +4     

Impacted Files	Coverage Δ
config/root_config.go	40.93% <66.66%> (+3.35%)	⬆️
config/custom_config.go	76.92% <76.92%> (ø)
remoting/zookeeper/client.go	0.00% <0.00%> (ø)
registry/zookeeper/listener.go	0.00% <0.00%> (ø)
remoting/zookeeper/listener.go	0.00% <0.00%> (ø)
config/config_center_config.go	50.48% <0.00%> (+0.48%)	⬆️
metadata/report/nacos/report.go	40.00% <0.00%> (+1.76%)	⬆️
cluster/cluster/failback/cluster_invoker.go	78.02% <0.00%> (+2.19%)	⬆️
protocol/dubbo/impl/hessian.go	34.37% <0.00%> (+3.14%)	⬆️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a2c0731...1eb4eb9. Read the comment docs.

[cjphaha]

👍

[LaurenceLiZhixin]

这里缩进是不是有问题？ @A-Wanderer

[LaurenceLiZhixin]

事实证明是这个问题，我认为这个pr可以merge了。相关rootConfig问题麻烦记录个issue。