Add on_conflict option for upsert

[ConnorVO]

Is your feature request related to a problem? Please describe.
I need to be able to define the column to check for conflict when upserting

Describe the solution you'd like
Add an on_conflict option like the js API

Describe alternatives you've considered
Currently I have to write my own upsert functionality to check if a row containing a value in a specific column exists and then updating if it does or inserting if not

[J0]

Hey @ConnorVO,

Thanks for the query - there's an open PR on postgrest-py but it's been a while. I'll transfer this issue to the postgrest-py repo so that you can be best notified when it gets merged.

Lmk if there are further queries

Jo

[J0]

Fixed by #142