Extend the devops DEFAULT_DEV_OPS_URL to include the protocol

[hemidactylus]

In the AstraDBOps, the current definition of

DEFAULT_DEV_OPS_URL = "api.astra.datastax.com"

makes it impossible to test against e.g. a local (http, not https) endpoint unless one tweaks the class internals.

Suggestion: refactor the default and the behaviour in the constructor to allow specifying any arbitrary URI. This would touch the above as well as the line in the constructor currently like this:

self.base_url = f"https://{dev_ops_url}/{dev_ops_api_version}"