Support grouping alerts by source (for freshness)

[acarter24]

Is your feature request related to a problem? Please describe.
I love the alerts for sources, but if you have multiple freshness criteria on separate tables in single source, you will get one alert per source table.

If the grouping functionality for tables could be extended to sources, that would be great

Describe the solution you'd like
Be able to specify grouping alerts by source in the meta config.

Describe alternatives you've considered

Additional context

Would you be willing to contribute this feature?
Python dev but no knowledge of elementary code base

[github-actions]

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