Support directed identity for non-interactive grants

[raducristianpopa]

Context

When requesting a grant, the client field represents the wallet address of the entity that makes the request. To address the privacy concerns for the in browser implementation of Web Monetization we want to support a directed identity approach for non-interactive grants.

More context: Open Payments User Wallet Directed Identity Document

Todos

• The client field should be an object that accepts the following properties: jwk or walletAddress (mutually exclusive - only one of them)
  • jwk should be constrained to the format:

    open-payments/openapi/wallet-address-server.yaml

    Lines 153 to 178 in f259f30

    	json-web-key:
    	type: object
    	properties:
    	kid:
    	type: string
    	alg:
    	type: string
    	description: 'The cryptographic algorithm family used with the key. The only allowed value is `EdDSA`. '
    	enum:
    	- EdDSA
    	use:
    	type: string
    	enum:
    	- sig
    	kty:
    	type: string
    	enum:
    	- OKP
    	crv:
    	type: string
    	enum:
    	- Ed25519
    	x:
    	type: string
    	pattern: '^[a-zA-Z0-9-_]+$'
    	description: The base64 url-encoded public key.

• Backwards compatibility for the client field - should be available to use in both formats (string and the new object) for some time to allow people to migrate.