clarify that support for Batch Issuance is mandatory

[Sakurann]

at least in the short-mid term, support for Batch Issuance is crucial for verifier-verifier unlinkability.

also minimum batch size needs to be defined. suggestion would be 10.

[jogu]

Discussed on today's WG call - possible rough text:

Some credentials will always identify the user when it is presented - batch issuance is not needed for these credentials. It also not needed for claims bound credentials (but this is not possible in HAIP currently, but we have an issue open to allow it). If verifier-verifier linkability needs to be prevented for a particular credential, batch issuance must be supported and used.

[jogu]

/duplicate #202