Colour options for tags on DAGS

**Description**

Following the useful feature of tags with https://airflow.apache.org/docs/apache-airflow/2.0.0/howto/add-dag-tags.html it would be great if there was an option here to choose different colours for the tags.

**Use case / motivation**

As per the description, it would be great to see options for colour choices when adding tags. This will be really useful for those with several DAGs that might have dependencies on an upstream DAG where the same colour can be added. This will help users see at a glance which category/type a DAG is under.

Whilst it's a small thing, I think it will be really beneficial to users and certainly make the UI more colourful.

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

**Related Issues**

Start with the DAG tags documentation linked in the issue and locate the Airflow web UI entry point that renders DAG tags. Check the existing tag behavior and related UI tests, then define the supported color choices and verify that selected colors appear consistently for tagged DAGs.

Written by the indexing model from the issue text.