Feat: Bearer Authorization

[kilianmh]

Added bearer-authorization and proxy-bearer-authorization.
When (and bearer-authorization basic-authorization) or
(and proxy-bearer-authorization proxy-basic-authorization)
an error is signaled.

Also added tests for error signalling and whether the authorization value appears appropriately in the header.

[stassats]

Is it substantially better than just using :additional-headers?

[kilianmh]

Bearer authorization is widespread nowadays, e.g. for oauth2 and api calls.

Is it substantially better than just using :additional-headers?

Adding new parameters makes it more convenient and less error prone (no typo in "Bearer ").

There is also a sanity check that basic-authorization is not supplied at the same time with
bearer-authorization. We could also add a check that gives an error when another
authorization header is present in :additional-headers, but that might impact performance more.