Add the entity class for xDS flow control functionality

[hanbingleixue]

What type of PR is this?

Feature

What this PR does / why we need it?

Add the entity class for Xds flow control functionality

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

All modified and coverable lines are covered by tests ✅

Flag	Coverage Δ	Complexity Δ
unittests	44.07% <100.00%> (+0.11%)	181.00 <0.00> (ø)

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

Files with missing lines	Coverage Δ	Complexity Δ
...ant/core/service/xds/entity/FractionalPercent.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...a/io/sermant/core/service/xds/entity/XdsAbort.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...nt/core/service/xds/entity/XdsCircuitBreakers.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...a/io/sermant/core/service/xds/entity/XdsDelay.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
.../io/sermant/core/service/xds/entity/XdsHeader.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...rmant/core/service/xds/entity/XdsHeaderOption.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
.../sermant/core/service/xds/entity/XdsHttpFault.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...t/core/service/xds/entity/XdsOutlierDetection.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
.../sermant/core/service/xds/entity/XdsRateLimit.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
...ermant/core/service/xds/entity/XdsRetryPolicy.java	100.00% <100.00%> (ø)	0.00 <0.00> (?)
... and 1 more

... and 39 files with indirect coverage changes