Validate endpoint connection info without saving it first

[mihaelabalutoiu]

Previously, the endpoint validation API only worked on stored endpoints (/endpoints/{id}/actions), so we had to save an endpoint before we could check its connection info. Invalid endpoints were persisted when validation failed.

This PR implements the following:

• Support validation bodies in key error messages:
  • allow format_keyerror_message to describe missing attributes of a validation request body.
• Validate endpoint connection info without saving it first:
  • move the validation action to /endpoints/actions and validate the platform and connection_info given in the request body, so an endpoint can be checked before it is saved.
• Updates the API reference, the policy docs and the unit tests.

Related PRs: coriolis-web python-coriolisclient

[mihaelabalutoiu]

Note: The integration tests fail with:

TypeError: EndpointManager.validate_connection() takes 2 positional arguments but 3 were given

This is expected: the integration tests install python-coriolisclient from its main branch, which still has the old validate_connection(endpoint) signature.
The CLI PR needs to be merged first, then the integration tests can be re-run.