Add Set Extra HTTP Headers keyword to change request headers mid-test #5115

Use case

Extra HTTP headers (auth tokens, correlation/tracing IDs, tenant selectors, feature-flag headers) can currently only be set once, at New Context time via extraHTTPHeaders. When a test obtains a fresh token mid-session — after login, after a token refresh, or when switching users — the only way to change headers today is to close the context and create a new one, which throws away all page state, cookies and open pages. Playwright allows updating the headers on a live context (or page) at any time; the library should too.

Proposed keyword / arguments

Set Extra HTTP Headers &{headers} *, scope=Context

• headers — free named arguments (or a dict) of header name → value; applied to all subsequent requests. Header values must be strings.
• scope — Context (default): headers apply to every page in the current context, including pages opened later; Page: only requests initiated by the current page (page-level headers are merged over context-level ones, per Playwright semantics).

New Context
New Page    ${URL}/login
# ... perform login, obtain a token ...
Set Extra HTTP Headers    Authorization=Bearer ${token}    X-Correlation-Id=${test_id}
Go To    ${URL}/dashboard

# Override for the current page only
Set Extra HTTP Headers    X-Experiment=variant-b    scope=Page

Playwright API

• browserContext.setExtraHTTPHeaders
• page.setExtraHTTPHeaders

Implementation notes

• New RPC in protobuf/playwright.proto carrying the header map plus a page/context flag.
• node/playwright-wrapper: resolve current page or context from the catalog, call setExtraHTTPHeaders.
• Python keyword (natural home: Browser/keywords/network.py or playwright_state.py), docs, atest asserting the header arrives server-side (test app echoes request headers) and that scope=Context covers a page opened after the call.

Backwards compatibility

Purely additive: a new keyword; the existing New Context extraHTTPHeaders= argument is untouched and simply provides the initial value the keyword can later replace.