xds client and util

[daizhenyu]

What type of PR is this?

Feature.

What this PR does / why we need it?

Tool class, client, and abstract xdshandler of the xds service discovery module

Which issue(s) this PR fixes？

Fixes #1515

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 37.26708% with 101 lines in your changes are missing coverage. Please review.

Flag	Coverage Δ	Complexity Δ
unittests	43.07% <37.26%> (+0.03%)	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
...n/java/io/sermant/core/service/ServiceManager.java	9.37% <ø> (ø)	0.00 <0.00> (ø)
...in/java/io/sermant/core/service/ServiceConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...plement/service/xds/entity/XdsServiceInstance.java	50.00% <50.00%> (+50.00%)	0.00 <0.00> (ø)
.../io/sermant/core/service/xds/config/XdsConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...mant/implement/service/xds/cache/XdsDataCache.java	70.00% <40.00%> (+70.00%)	0.00 <0.00> (ø)
...ment/service/xds/utils/XdsProtocolTransformer.java	84.84% <84.84%> (ø)	0.00 <0.00> (?)
...ermant/implement/service/xds/client/XdsClient.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...mant/implement/service/xds/handler/XdsHandler.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)

... and 7 files with indirect coverage changes