Set `client_credentials` as the default OAuth2 grant type.

**Is your feature request related to a problem? Please describe.**

Currently, the go client doesn't support setup OAuth2 params without the grant type to be specified.
The broker only supports the `client_credentials` grant type. It would be better to make this the default grant type for the Go client. This change would align the Go client's behavior with that of the Java client.

**Describe the solution you'd like**
Set `client_credentials` as the default OAuth2 grant type.

**Describe alternatives you've considered**
None yet

**Additional context**
None

Start by locating the Go client's OAuth2 parameter setup and the grant-type default, then compare its behavior with the Java client mentioned in the issue. The work is done when client setup succeeds without an explicitly supplied grant type and defaults to client_credentials, with relevant existing tests updated or added.

Written by the indexing model from the issue text.