Auto merge of #10111 - DDEFISHER:master, r=jdm

add auth cache to resources + set auth header from it if url does not have creds

initial attempt of

- in resource_thread.rs, define an HTTP authorization cache storage (username, password, URL) and instantiate it like the cookie_storage member (inside an Arc<Rwlock<>> value, to enable sharing it between threads)
- in modify_request_headers in http_loader.rs, implement the remaining pieces of step 12(13?) of the appropriate specification using this new authorization cache.

 for the NCSU student project Implement HTTP authorization UI and persistent sessions.

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