allow issuance of multiple cred at same revocation index

[Zzocker]

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This PR proposes an option for the issuer to specify a cred_rev_id (index in revocation registry) when a credential is issued.This gives more control for an issuer to manage revocation registry, a few examples can be

• Alice issues credentials with same cred_rev_id multiple times to Bob, who wants the same credential to be stored in multiple wallets, In this case Alice can revoke a credential of Bob by revoking a single index, this allows Alice to create a better controller logic for revocation registry
• Alice issues credentials with same cred_rev_id to multiple Holders e.g. Coupons

@DaevMithran

Signed-off-by: Pritam Singh [email protected]

[DaevMithran]

@andrewwhitehead can you review this ?