Plugin: improve ICS plugin

[samber]

Add sources:

• watch from file
• watch from url

Add sinks:

• write to file

Add operator:

• serialize (or different name ?)
• unserialize (or different name ?)
• validate
• filter by participant
• filter by time window
• filter by ...
• dedup (multiple ics from multiple participants could showcase the same event)

I think the current source operators should return a custom struct instead of *ics.VEvent

[dagbolade]

Hi can this issue be assigned to me?