AIP-38 | Merge Backfill and Trigger Dag Run modals

[bbovenzi]

Body

Problem:

• Create backfill is too hidden away, missing dag run conf and ultimately, is just a version of triggering a run

Solution:

• Add an option to toggle between a regular dag run trigger or a backfill trigger in the same modal. Kind of like how we have materialize asset and create asset event in the same modal.

Committer

• I acknowledge that I am a maintainer/committer of the Apache Airflow project.

[guan404ming]

Hi, @bbovenzi I'm interested in this issue, could I help with this? Thanks~