feat: reduce affordances provided by `IParties`

This is in preparation for supporting a local development mode of register with reduced features but no dependency on Altinn 2. This commit creates a new Altinn.Register.Core project and moves and renames the IParties interface to it. It also updates the interface with more idiomatic naming of methods and ensure that they all take in CancellationTokens. Lastly, it reduced the affordances provided by the interface by limiting the size of it's outbound contract while also relaxing the input constraints.

Description

Related Issue(s)

{issue number}

Verification

[ ] Your code builds clean without any errors or warnings
[ ] Manual testing done (required)
[ ] Relevant automated test added (if you find this hard, leave it and we'll help out)
[ ] All tests run green

Documentation

[ ] User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)

Altinn / altinn-register