Add GH Action tests for different backends independently

[pjbull]

Currently, our test suite installs all of the possible dependencies for every cloud provider.

This means that errors in the code that occur when only one of the cloud provider is installed are not caught by CI. For example, see the report in this issue.

We should add a separate step for each "extra" that installs in a fresh environment and does a basic import check that everything works as expected even without the optional dependencies.

[pjbull]

closed in #302