Support limiting number of data sources

[jen-huang]

For some packages like Endpoint, agent configs can only support 1 data source from that package. Those restricted data source definitions should have a property to flag that for down stream consumers.

Property name ideas:
no_conflict: boolean (if true, up to 1 of same data source)
no_duplicate: boolean (same as above)
self_limit: number (if specified, up to the limit value of same data source)

(...naming is hard 😅)

[ruflin]

What about multiple: false? I would prefer we stick with a boolean for now as I don't see a use case where we would say 42 are possible.

[jen-huang]

+1 on that naming and boolean @ruflin

[ruflin]

I opened #361 for this