[Nacos 3.0] Add tests and documents for xDS feature

[KomachiSion]

Nacos 3.0 support xDS protocol by OPSS in #10389 and #8313. But no enough tests to ensure feature right and enabled in future changes.

And current document only include the CDS and EDS in doc. This doc is with old title and no detail usage description for xDS feature.

So nacos community hope someone can help to add enough tests for feature and add usage document with right title and detail description.

sub tasks following:

• Add tests for xDS.
• Enhance and modify the document about xDs features.

[godhth]

please assign to me