xdsclient: export genericResourceTypeDecoder

[easwars]

This is a follow-up from #8405.

Exporting this type will allow the registration of other resource type implementations. Currently this is in an internal package, and we will anyway get rid of this code once we migrate all our resource type implementations to use the new generic client API directly. This is only a temporary hack.

RELEASE NOTES: none

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.39%. Comparing base (15299cc) to head (11807a1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8406      +/-   ##
==========================================
+ Coverage   82.29%   82.39%   +0.10%     
==========================================
  Files         414      414              
  Lines       40420    40420              
==========================================
+ Hits        33263    33305      +42     
+ Misses       5789     5757      -32     
+ Partials     1368     1358      -10     

Files with missing lines	Coverage Δ
...nal/xdsclient/xdsresource/cluster_resource_type.go	78.37% <100.00%> (ø)
...l/xdsclient/xdsresource/endpoints_resource_type.go	76.47% <100.00%> (ø)
...al/xdsclient/xdsresource/listener_resource_type.go	86.44% <100.00%> (ø)
...ds/internal/xdsclient/xdsresource/resource_type.go	59.67% <100.00%> (ø)
...dsclient/xdsresource/route_config_resource_type.go	76.47% <100.00%> (ø)

... and 22 files with indirect coverage changes

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