feat: add access-identity to auth-server.yaml

[desimmons]

Changes proposed in this pull request

This PR is a proposal for a new grant type: access-identity

Context

As part of the Open Payments community call, the need for a user-authentication grant was identified by two participants. One of the actions from the latest call was to create proposals. This is one such proposal.

TODO

• Update resource-server.yaml
• Create basic example docs

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[raducristianpopa]

Based on the discussion from the Open Payments Community call, this would also require an additional property that will need to be provided - the user wallet address. This way, the wallet provider can validate if the connected user (on their side) is the owner of the wallet address when the user is redirected to the IdP page.