Use client authn on device auth request

[nsklikas]

According to https://datatracker.ietf.org/doc/html/rfc8628#section-3.1:

The client authentication requirements of Section 3.2.1 of [RFC6749]
apply to requests on this endpoint, which means that confidential
clients (those that have established client credentials) authenticate
in the same manner as when making requests to the token endpoint, and
public clients provide the "client_id" parameter to identify
themselves.

This PR introduces client authentication on the device auth request. Same mechanism used for client authentication on the token endpoint is used on the device auth request as well.

The logic for making the device auth request was moved to the internal package similar to the token request logic, so that we can re-use the lookupAuthStyle cache.

Fixes #685

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