feat(ockam/credential): Set the vault argument value name and add doc…

… string

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.

implementations/rust/ockam/ockam_command/src/credential/issue.rs

@@ -28,7 +28,8 @@ pub struct IssueCommand {
     #[arg(short, long = "attribute", value_name = "ATTRIBUTE")]
     pub attributes: Vec<String>,
 
-    #[arg()]
+    /// Name of the Vault that will be used to issue the credential.
+    #[arg(name = "VAULT_NAME")]
     pub vault: Option<String>,
 
     /// Encoding Format