feature request: carryforward flags

[mrexox]

Hi!

Thank you for maintaining this coveralls integration! I'd like to request a small feature.

Coveralls now supports carryforward flags which allow to carry missing jobs from previous builds by flag names. It would be great to have this feature in your integration!

It simplifies some flows with multiple jobs within one build as described here.

In a few words - it is an extra parameter for this endpoint - /webhook&carryforward="flag1,flag2,flag3,...". If the value is empty it is ignored.

You can use coveralls-reporter as a reference. Ask me for any details about this feature, I'll be glad to help 🙏

[TheKevJames]

Thanks for reporting! Looks fairly straightforward, definitely would be nice to support here. Marking as help wanted in case anyone wants to submit a PR to tackle this.