Add credential support for Azure Data Factory in CLI

**Related command**
az datafactory

**Describe the solution you'd like**
support for creating, deleting, getting, and listing credentials by factory, similar to existing REST API support: https://learn.microsoft.com/en-us/rest/api/datafactory/credential-operations

**Describe alternatives you've considered**
REST API support exists, but is a bit cumbersome to implement. Alternatively, the portal can be used to create creds manually; however, that does not work well for cross-subscription managed identities (MIs), as subscription-level access is required to view MIs in another subscription.

**Additional context**
This will assist in providing a more familiar interface to create credentials in ADF via the CLI, as well as the ability to easily attach cross-subscription MIs by providing just the resource ID.

If possible, I would be happy to contribute to this feature.