feat: Set up config check for connection validity

[elefeint]

Fivetran framework expects a list of connection tests to be returned from the ConfigurationForm endpoint. It will then call the Test endpoint in order for each of the tests specified.

Given that the token is already a required property on the Fivetran side, for the MotherDuck destination there is only one useful check -- "can the user authenticate?". The check already existed, but it was not making use of the TestRequest.name() field.

I thought about making an unknown test name log a warning and succeed, but this will just hide potential integration problems, so I ended up making it an error instead.

Fixes #5

For reference, here is an example of configuration tests for a Github source:

[guenp]

Looks good to me, just had one suggestion