Decide on sync vs async DataFrame methods

### Is your feature request related to a problem or challenge?

The current design is using synchronous execution. In https://github.com/pgwhalen/datafusion-java everything seems to be async.

Let's decide how we want to handle this before we add too much more code.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_