Feature: Pass generic additional parameters back through Login (#55)

* Refactor DPoP Signing to correctly track nonces per origin

This also ensures we clearly signal to the DPoP request method whether we are talking to an authorization server or a resource server (since these give different OAuth Error responses).

The test coverage has increased dramatically here.

* Remove mutability in buildProof

* Fix Bluesky token error handling

This is actually generic OAuth token response logic, but the package is currently structured to make this bluesky specific.

* Fix PAR error handling

* Allow passing along generic additional parameters with Login

This allows apps using the Bluesky client to retrieve the DID for the authenticated account from the Login.

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Co-authored-by: Anna M <anna@germ.network>