Add a factory method for Issuer for wallet-initiated case

[babisRoutis]

In the case of wallet-initiated issuance, the wallet must know out-of-band:

• The id of the credential issuer, and
• One or more credential configuration identifiers that the issuer supports

This is equivalent of having a credential offer, that uses authorization code flow (without issuer_state).

Library already supports this, yet caller has to manually,

• resolve credential issuer and authorization server meta-data and
• assemble a credential offer as described above

There could be a factory method for Issuer that does automates this.