xdsclient: remove unexported method from ResourceData interface

[easwars]

Having an unexported method in the ResourceData interface is not very useful since it has other exported methods. Implementations are currently forced to embed this interface to satisfy it. But if a breaking change is made to the interface, implementations will panic at runtime if the newly added method is called (since they embed the interface and therefore satisfy the interface, but don't have an implementation for the newly added method).

Removing the unexported interface results in implementations no longer having to embed this interface. Instead, they can rely on compile time type assertions to ensure that their implementation satisfies the interface, and therefore their build will fail at compile time if a breaking change is made to the interface (instead of a runtime panic).

RELEASE NOTES: none

[easwars]

@purnesh42H : FYI

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.88%. Comparing base (274830d) to head (8ef24df).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7835      +/-   ##
==========================================
- Coverage   81.90%   81.88%   -0.02%     
==========================================
  Files         374      374              
  Lines       37978    37978              
==========================================
- Hits        31105    31099       -6     
+ Misses       5579     5578       -1     
- Partials     1294     1301       +7     

Files with missing lines	Coverage Δ
...ds/internal/xdsclient/xdsresource/resource_type.go	100.00% <ø> (ø)

... and 18 files with indirect coverage changes

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