Credential vending for read only credentials

Hi and thanks a lot for this extension, it has been extremely useful for me!

One missing piece for me is that several tables that I'd like to query are Unity Catalog managed Delta tables. This means that the credential vending will only work for [read-only credentials as noted in the databricks documentation](https://docs.databricks.com/aws/en/external-access/credential-vending#table-credential-vending).

Now the extension is vending (hardcoded) `READ_WRITE`-credentials which will fail in that case. A fix that would work perfectly for me is to do only `READ`-credentials when the catalog is attached in `READ_ONLY`-mode. As there should be no need for `READ_WRITE`-credentials in this case anyway, I feel like this shouldn't break any existing behavior.

I'll open a PR for this shortly, feel free to comment here or there if there are any comments or changes you'd like!