[SWSyntheticResponse] Add a flag to bypass the opt-in header check

The Service Worker Synthetic Response feature currently requires the
server to send a Service-Worker-Synthetic-Response: ?1 header to enable
storing the response for future navigations. This makes local
development and testing difficult, as it requires setting up a server to
provide the specific header.

This CL introduces a new feature flag,
ServiceWorkerBypassSyntheticResponseHeaderCheck, to simplify this
workflow. When enabled, the browser bypasses the opt-in header check,
allowing any successful response to be stored for synthetic use.

Additionally, this flag includes a parameter, ignored_headers, which
allows specifying a comma-separated list of headers to ignore during the
header consistency check. This is useful for ignoring headers like
Content-Security-Policy or Alt-Svc during tests.

Usage examples:
 - To bypass the opt-in header check:
  --enable-features=ServiceWorkerBypassSyntheticResponseHeaderCheck

 - To also ignore specific headers during the consistency check:
  --enable-features=ServiceWorkerBypassSyntheticResponseHeaderCheck:ig
  nored_headers/content-security-policy,alt-svc

Bug: 352578800
Change-Id: Ib116a5c22d6642df6465561987556a5dc4a2974e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6695612
Reviewed-by: Shunya Shishido <sisidovski@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Auto-Submit: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1482087}