feat(dash): Enable Stateful Mode and rsq Corruptions for DASH

[Kajlid]

• Imported STATEFUL and newState utilities for DASH (segment.ts, dashManifestUtils.ts and dashManifestUtils.test.ts) to support stateful mode.
• Ensured rsq corruptions are possible for DASH when the proxy is running in stateful mode, similar to how rsq corruptions are enabled for HLS.

This should solve the issue #63: Support for relative corruptions (rsq) for DASH live streams.

[Nfrederiksen]

It looks good. The feature is indeed implemented and behaves as expected 👍
Only a few thing I'd need before approving.

1. .history can be included in .gitignore as well. No need to include it in the main branch.
2. Now sure in what update it got missing but add back import { URLSearchParams } from 'url'; in utils.ts,
   just to be consistent in the project as it is now currently using the global URLSearchParams in that script when without the import.
3. I like the handling of the case when a relative sequence corruption is attempted but the proxy is not in a stateful mode in config.ts.
   And although it is originally outside of the scope of the original issue Support for relative corruptions (rsq) for DASH live streams #63 , it would be much appreciated if
   a unit test could be added for that case in config.test.ts method getAllManifestConfigs.
   It needs to test if isDash AND !STATEFUL, and also case if !isDash AND !STATEFUL. This would be super nice to have in place :)
4. Lastly, after all other points are completed. One final task is to update the READme file with the new feature and an example url. Just follow other examples. :)

[Kajlid]

Hi Nicholas!
Thank you for your answer and for giving feedback in such a structured and understandable way :). I have updated the code with the changes you suggested, including the new unit test and updated README.

I did not quite understand the first sentence of point 3, since the code I added in config.ts was inspired from the similar if statement above, giving an error message if !STATEFUL and !isDash, so I added a similar error message for !STATEFUL and isDash. This is also what the new unit test in config.test.ts is checking for, those error messages. In other words: I added code in config.test.ts now, but did not understand if I also need to make some change in config.ts.

Thanks again for the feedback and please get back if I need to make additional changes.

[Nfrederiksen]

This looks good to me.