Clarify key selection for client_metadata jwks

[jogu]

Feedback from the ISO mdl DC API interop preparation:

The way that keys should be passed in client_metadata / the way that the key in the client_metadata jwks to use for encryption is selected is unclear.

Some verifiers are including use: enc in the encryption jwk, some are not. Some wallets were requiring a key with use:enc, some were not.

I think there's perhaps a general assumption currently that a "normal" style key selection algorithm would be used, something like described in FAPI: https://openid.net/specs/openid-financial-api-part-2-1_0.html#duplicate-key-identifiers but we should be explicit about the expected behaviour.