Allow marking custom headers as sensitive on redirects

[floitsch]

Custom credentials such as X-Api-Key currently follow cross-origin redirects. Add --sensitive-headers to both Client constructors so callers can name additional headers to strip whenever the scheme, host, or port changes. Matching is ASCII case-insensitive; same-origin redirects retain credentials, and credentials removed during a chain are never restored. The configuration list is copied and caller-owned request headers remain unchanged.

This is an additive policy: ordinary custom headers continue to be forwarded. Applications must list their custom credential headers (for example, http.Client network --sensitive-headers=["X-Api-Key"]) or disable redirect following. The three standard credential headers remain unconditionally protected. This avoids introducing a restrictive allowlist that would break existing custom-header forwarding.

Follow-up to #174: #174 (comment).

Validation on Toit v2.0.0-alpha.199:

• 19/19 local tests passed with ctest --test-dir build -j4 --output-on-failure -E 'google|webdriver', configured with ENABLE_HTTPBIN_TESTS=OFF.
• New regressions cover both constructors, GET/POST/WebSocket redirects, 303/307 responses, host/port/scheme changes, duplicate values, mixed-case configuration, configuration mutation, caller ownership, and return-to-origin chains.
• Removing the additional-header filter makes the new regression fail.
• Source and test analysis and git diff --check passed.

Live Google, browser, and httpbin integration tests were not run locally. This PR targets merged main and can be reviewed independently of the downgrade-policy follow-up.

[floitsch]

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