CLI interface to expose unified REST interface

[dovydas-an]

Develop alpha version of a CLI interface that exposes the functionality of unified REST interface. (#26).

[Jonas-Puksta-Sensmetry]

Implemented a check backend which just calls the check library REST API here 850dc53. This is needed since we may want to use the CLI interface from somewhere where we don't have direct access to the backend (e.g. in GitHub pipelines)

[victor-linroth-sensmetry]

Started implementing a CLI in commit 06d39b1.

[victor-linroth-sensmetry]

Continued extending the CLI in commits 87a9bce, f103ade, bc99fb2, 49c1af8. CLI can now generate a local configuration where you can set which backends to use, with the possibility of using multiple different backends. Supports listing and using check templates, as well as creating, deleting and listing checks. Updating checks remains to be implemented, as does configuring individual settings of backends.

[victor-linroth-sensmetry]

No major new features but I did some general restructuring and polishing of the CLI to make it a bit more usable, specifically the configuring of the backends. I have also been working on making the error handling a bit better.

[victor-linroth-sensmetry]

Improved error handling. No longer crashes when failing to connect to backend

[tilowiklundSensmetry]

#26 (comment)