[actix-web-httpauth] Custom error response when BearerAuth extrator is used

I'd like to specify a custom error handler for `BearerAuth` extractor, possibly via its `Config`.
This is already supported by `HttpAutentication` middleware (https://github.com/actix/actix-extras/issues/156), but the middleware seems to be independent from the extractor.

Start by locating the Rust implementation of the BearerAuth extractor and its Config, then compare it with the existing HttpAuthentication middleware error-handler path. Done means the extractor can accept and invoke a custom error handler through its configuration, with the behavior verified by the relevant tests.

Written by the indexing model from the issue text.