Support IOBuffer writes for HTTP streams

[quinnj]

Summary

• add Base.unsafe_write for HTTP.Stream by copying pointer data and delegating through the existing stream byte write path
• cover IOBuffer writes for server stream responses and HTTP.open request bodies

Tests

• julia --project=. -e 'include("test/http_server_http1_tests.jl")'\n- julia --project=. -e 'include("test/http_client_tests.jl")'\n\nCo-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.86%. Comparing base (6022a9f) to head (de5d050).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1300      +/-   ##
==========================================
+ Coverage   86.83%   86.86%   +0.03%     
==========================================
  Files          28       28              
  Lines       10799    10803       +4     
==========================================
+ Hits         9377     9384       +7     
+ Misses       1422     1419       -3     

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