fix host rewrite in frontend-gray

[johnlanni]

Ⅰ. Describe what this PR did

In Envoy, rewriting the host needs to be achieved by modifying the :authority pseudo-header.

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.63%. Comparing base (ef31e09) to head (2a53d09).
Report is 288 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1747      +/-   ##
==========================================
+ Coverage   35.91%   43.63%   +7.72%     
==========================================
  Files          69       76       +7     
  Lines       11576    12358     +782     
==========================================
+ Hits         4157     5392    +1235     
+ Misses       7104     6629     -475     
- Partials      315      337      +22     

see 70 files with indirect coverage changes