refactor(http): move unstable APIs (header, method, signed-cookie)

[kt3k]

This PR moves header, method, and signed-cookie APIs to unstable-* paths.

I'll work on route in another PR, as it seems needing further care because of the dependency from deno init --serve

part of #5920

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (635a886) to head (b465f49).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5938      +/-   ##
==========================================
- Coverage   96.32%   96.32%   -0.01%     
==========================================
  Files         483      483              
  Lines       39415    39415              
  Branches     5838     5838              
==========================================
- Hits        37968    37965       -3     
- Misses       1405     1408       +3     
  Partials       42       42              

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