internal/buffer: make Unbounded buffer use generic type

[eshitachandwani]

Refactors internal/buffer.Unbounded (and its clone under internal/xds/clients) to use generics ([T any]) instead of any. This improves type safety and removes type assertions across consumers.

RELEASE NOTES: None

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.05%. Comparing base (8d66cfc) to head (ea235e1).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9196      +/-   ##
==========================================
- Coverage   83.27%   83.05%   -0.22%     
==========================================
  Files         419      419              
  Lines       33863    33886      +23     
==========================================
- Hits        28198    28144      -54     
- Misses       4252     4302      +50     
- Partials     1413     1440      +27     

Files with missing lines	Coverage Δ
balancer/rls/balancer.go	86.55% <100.00%> (ø)
internal/buffer/unbounded.go	100.00% <100.00%> (ø)
internal/grpcsync/callback_serializer.go	100.00% <100.00%> (ø)
internal/xds/balancer/outlierdetection/balancer.go	89.46% <100.00%> (ø)
...l/xds/balancer/outlierdetection/subconn_wrapper.go	91.22% <ø> (ø)
internal/xds/balancer/priority/balancer.go	87.06% <100.00%> (ø)
internal/xds/clients/internal/buffer/unbounded.go	100.00% <100.00%> (ø)
...s/clients/internal/syncutil/callback_serializer.go	100.00% <100.00%> (ø)

... and 27 files with indirect coverage changes

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