add tests to store funcs + add source-based sorting + coordinator URL validation

[sonroyaalmerol]

Context

• Majority of the PR is to add automated tests on most of the store functions.
• Also adds source as an option for SORTING_KEY (Disable sorting altogether? #217)
• This also adds an additional step in the stream coordinator which validates the requested URL and uses the title of the stream instead as its identifier. (related to Channels Not Playing Right #227)

Choices

Test instructions

Checklist before requesting a review

• I have performed a self-review of my code
• I've added documentation about this change to the README.
• I've not introduced breaking changes.

[github-actions]

The Docker image for this pull request has been built and pushed to GHCR.

Image URL: ghcr.io/sonroyaalmerol/m3u-stream-merger-proxy:pr-228