fix(ockam/credential): set the vault argument value name on ockam credential issue

[AshwinKul28]

Set the VAULT_NAME argument in the Ockam credential issue module and added a doc string describing the name of the Vault to be used for issuing credentials.

This change improves code readability and documentation for Vault usage.

Fixes #6386

Current behavior

The vault argument value name on ockam credential issue is not set.

Proposed changes

The vault argument value name on ockam credential issue changed to VAULT_NAME and a doc string changed with Name of the Vault that will be used to issue the credential.

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[AshwinKul28]

Created this PR - #6419 to get added to the contributors' list. Once added, I can check the last option in the above comment.

[adrianbenavides]

Looks good @AshwinKul28 , will get merged soon! 🚀

Thank you for your contribution 🙏