internal/transport: validate balancer and address metadata

[buger]

Fixes #9199

Validate metadata supplied by balancer PickResult.Metadata before merging it into the outgoing context, and validate resolver/address metadata before converting it to HTTP/2 header fields.

The tests cover invalid address metadata at the transport boundary and invalid balancer metadata through an end-to-end RPC path.

RELEASE NOTES:

• transport: Invalid metadata supplied by balancers or resolver addresses is rejected before request headers are created.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.22%. Comparing base (d74861e) to head (1155fc0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9203      +/-   ##
==========================================
+ Coverage   83.21%   83.22%   +0.01%     
==========================================
  Files         420      420              
  Lines       34008    34012       +4     
==========================================
+ Hits        28299    28307       +8     
- Misses       4275     4278       +3     
+ Partials     1434     1427       -7     

Files with missing lines	Coverage Δ
internal/transport/http2_client.go	92.80% <100.00%> (+0.39%)	⬆️
stream.go	82.22% <100.00%> (+0.26%)	⬆️

... and 21 files with indirect coverage changes

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