(bug) Utilize entropy in anoncreds VCX when appropriate

[gmulhearn]

Currently our Anoncreds trait and anoncreds-rs implementation is hardcoded to take in prover_did instead of entropy. This is a legacy method - it's used in "indy" anoncreds exchanges, but modern anoncreds exchanges prefer entropy.

This is particularly problematic as anoncreds-rs enforces this a bit with some validation, e.g. "if using modern credential-definition-identifier (qualified), then entropy must be used instead of prover did".

However it may not be as simple as permanently switching to usage of entropy instead of prover_did, as "indy" anoncreds exchanges might be expecting the presence of prover_did...

This should be investigated and determine a suitable approach. Some ideas:

• choose entropy vs prover_did on behalf of the consumer based on the type of identifier they provide (legacy or not)
• allow a config/flag to be passed in to choose between entropy or prover_did - this way, protocols utilizing "indy" exchanges can opt-in to using prover_did.

related issue (see thread): openwallet-foundation/acapy#3369