Move shared DPoP code into IdentityModel

[josephdecock]

The DPoPExtensions are shared between AccessTokenManagement and OidcClient. We should move the code into IdentityModel to simplify maintenance.

We should also look at the other DPoP code to see if more than just the extensions are shared.

This is a breaking change because the apis in question are public.

[josephdecock]

As part of this work, I'd also like to experiment with changing the cross mono-repo references from package references to project references. The implication of this is that when we release updated IdentityModel, that flows automatically to access token management and oidcclient.