Robons 77 http headers support

[robons]

• Moved from using Jenkins to GitHub Actions for PR + Master builds (were already using it for releases).
• Refactored code to use the Either class instead of exceptions - this helps ensure we're more robustly covering all edge cases -- I've put a lot more Metadata error messages in to the code base as a result.
• Started mocking (relevant) HTTP requests from the cucumber tests so that we can test the HTTP header support.
• Started to split up some of the larger classes to make the code base a bit more maintainable/comprehensible.

[github-actions[bot]]

ubuntu-latest test results

0 tests   0 :heavy_check_mark:  0s :stopwatch: 0 suites  0 :zzz: 0 files    0 :x:

Results for commit 28c5ebd6.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

Unit and Cucumber Test Results

486 tests   486 :heavy_check_mark:  31s :stopwatch:     8 suites      0 :zzz:     8 files        0 :x:

Results for commit 2d48f52d.

:recycle: This comment has been updated with latest results.

[robons]

Replaced by #108