Add optional justification when pausing a DAG

**Description**

It would be great to have a way to add a justification when pausing a DAG.

This could be also shown on when a DAG is paused to understand why the DAG is not active.

**Use case / motivation**

We use a lot of DAGs on a Airflow project right now and every time a DAG needs to be paused we open a issue on our issue tracker and communicate with the rest of the team about this change. This is done totally outside of Airflow right now and it's hard to keep track of why a given DAG on a given environment is paused and if it's supposed to be kept that way.

There should be a optional field for adding a justification when pausing a DAG. If present this field could also be used for showing more info on the DAG list when a DAG is paused.

**Are you willing to submit a PR?**

Yes, but would probably need some help.

**Related Issues**

N/A

No files, tests, or entry points are named in the issue. Start by tracing how DAG pause state is stored and how paused DAGs are shown in the DAG list; done means an optional justification can be recorded when pausing and is visible wherever the issue requires it.

Written by the indexing model from the issue text.