internal/xdscache: add v3 package

[skriss]

Depends on #3120
Depends on #3118

Commit 1486fda is the only one with actual changes.

[codecov]

Codecov Report

Merging #3121 (fb5f09f) into main (3e0cafd) will increase coverage by 1.68%.
The diff coverage is 82.89%.

@@            Coverage Diff             @@
##             main    #3121      +/-   ##
==========================================
+ Coverage   74.53%   76.21%   +1.68%     
==========================================
  Files         112      117       +5     
  Lines        7045     7577     +532     
==========================================
+ Hits         5251     5775     +524     
+ Misses       1686     1672      -14     
- Partials      108      130      +22     

Impacted Files	Coverage Δ
internal/xdscache/v3/endpointstranslator.go	70.62% <70.62%> (ø)
internal/xdscache/v3/route.go	81.89% <81.89%> (ø)
internal/xdscache/v3/cluster.go	87.87% <87.87%> (ø)
internal/xdscache/v3/secret.go	89.74% <89.74%> (ø)
internal/xdscache/v3/listener.go	91.84% <91.84%> (ø)
... and 8 more

[stevesloka]

/lgtm