Allow custom config fetcher + include CF ray ID

[adams85]

Describe the purpose of your pull request

Adds a new option named configFetcher, which allows consumers to choose the config fetcher implementation to use or to provide their own implementation (useful for testing primarily).

Plus, makes sure that Cloudflare Ray ID is included into log messages when the HTTP response is not successful. (We'll need some further CORS configuration to make this work in browser targets).

Also,

• corrects the API surface area of the main entry point,
• fixes a few minor bugs.

Related issues (only if applicable)

https://trello.com/c/Fdzvind3
https://trello.com/c/iCm8OlJv

How to test? (only if applicable)

n/a

Security (only if applicable)

n/a

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

[sonarqubecloud]

Quality Gate failed

Failed conditions
73.8% Coverage on New Code (required ≥ 80%)
4.1% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube Cloud