feat: JSON LD implementation in issuing credentials

[ghost]

What?

• This PR depicts the issuance of JSON LD credentials.

Why?

• Earlier, the issuance of credentials was dependent on the ledger. So as to make it ledger-independent, JSON LD implementation was necessary.
• This approach can be used as the basic feature to be given for any organization opting for CREDEBL subscription.

How?

• Updated createOffer API so as to make it issue JSON LD V2 credentials along with V1.
• Added getW3cCredentialById API so that the agent will be able to fetch JSON LD credential stored in the wallet.
• Added getW3CCredentials API so that the agent will be able to fetch all JSON LD credentials stored in the wallet.