[Feature] tag / keyword for feed sources

**Is your feature request related to a problem? Please describe.**
I don't know if the feature is already in FreshRSS or whether if it's me who can't find it..

**Describe the solution you’d like**
In addition to the categories I'd like to tag feed sources for additional filing possibilities : my categories are currently typically related to content theme (eg web / local / travel etc )
and I'd like to add distinctive tags/keywords related to the type of media (eg newsletter / podcast / video, etc)

Is this possible ?

**Describe alternatives you’ve considered**
None since feed sources can't be duplicated

**Additional context**

FreshRSS version 1.21.1-dev

Thanks

dd