xds base class and interface

[daizhenyu]

What type of PR is this?

Feature.

What this PR does / why we need it?

Xds ServiceDiscover Module base class and interface.

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 0% with 63 lines in your changes are missing coverage. Please review.

Flag	Coverage Δ	Complexity Δ
unittests	43.03% <0.00%> (?)	181.00 <0.00> (?)

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> (?)
...in/java/io/sermant/core/service/ServiceConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...mant/implement/service/xds/cache/XdsDataCache.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...src/main/java/io/sermant/core/utils/FileUtils.java	14.92% <0.00%> (ø)	0.00 <0.00> (?)
.../io/sermant/core/service/xds/config/XdsConfig.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...plement/service/xds/entity/XdsServiceInstance.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)