Defining sentry event custom tag in airflow.cfg

**Description**

Requiring for defining sentry event custom tag in airflow.cfg

**Use case / motivation**

Hi
If we have 1 project in sentry for airflow and i we 3 sites with airflow (dev,stage,prod) we need to be able to set the custom tag in the airflow config in the sentry section for sentry to be able to distinguish where the event came from

For example:
```
[sentry]
sentry_dsn = https://bdfe3b8a5c0e4xjkxjxxf21@sentry.mydomain.com/16
sentry_custom_tag = stand = develop
```
Now we have only this tags: https://airflow.apache.org/docs/stable/errors.html#tags