Enable loading model with references from GitHub

[atextor]

Description

With the changes in this PR, samm-cli commands no longer only take a local file
name as input for the Aspect Model, but also allow GitHub URLs and Aspect Model
URNs. Resolution from a given GitHub repository can be configured as a
resolution strategy, so it is now also possible to e.g. have an Aspect Model as
a local file that refers to elements hosted on GitHub (via usage of their
namespace; the resolution strategy will then figure out the location of the
corresponding file and retrieve it).

Fixes #474, fixes #664

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[Yauhenikapl]

LGTM!

[github-actions]

Release v2.9.7 addresses this.

[github-actions]

Release v2.9.7 addresses this.