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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