Add plugin handler and configuration classes

[hanbingleixue]

What type of PR is this?

Feature.

What this PR does / why we need it?

Add plugin handler and configuration classes

1. Add the configuration class of dynamic configuration management.
2. Add the plug-in processing class to verify whether a configuration item belongs to the current plug-in and parse the application name, service name, and environment name from the configuration item information.

Which issue(s) this PR fixes？

Fixes #1528

Does this PR introduce a user-facing change?

No

Checklist

• Make sure there is a GitHub_issue related with this PR before you start working on it.
• Make sure you have squashed your change to one single commit.
• GitHub Actions works fine in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 70.46632% with 57 lines in your changes are missing coverage. Please review.

Flag	Coverage Δ	Complexity Δ
unittests	43.10% <70.46%> (+0.21%)	181.00 <0.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ	Complexity Δ
...va/io/sermant/backend/common/conf/CommonConst.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...ant/backend/handler/config/OtherPluginHandler.java	85.71% <85.71%> (ø)	0.00 <0.00> (?)
...nt/backend/handler/config/PluginConfigHandler.java	93.75% <93.75%> (ø)	0.00 <0.00> (?)
...ckend/handler/config/FlowControlPluginHandler.java	88.23% <88.23%> (ø)	0.00 <0.00> (?)
...d/handler/config/TagTransmissionPluginHandler.java	80.00% <80.00%> (ø)	0.00 <0.00> (?)
...kend/handler/config/LoadbalancerPluginHandler.java	66.66% <66.66%> (ø)	0.00 <0.00> (?)
...t/backend/handler/config/RemovalPluginHandler.java	76.47% <76.47%> (ø)	0.00 <0.00> (?)
...d/handler/config/ServiceRegistryPluginHandler.java	63.63% <63.63%> (ø)	0.00 <0.00> (?)
...andler/config/SpringBootRegistryPluginHandler.java	63.63% <63.63%> (ø)	0.00 <0.00> (?)
...a/io/sermant/backend/entity/config/PluginType.java	76.19% <76.19%> (ø)	0.00 <0.00> (?)
... and 4 more

... and 2 files with indirect coverage changes