Support composite IDs from Prisma

[tvvignesh]

Hi. Great work with the tool 👍🏼 I am thinking of exposing this to our business. Was trying this out and noticed @@id is not suported as of now.

If I want to define a key as @@id([field1, field2]) as defined here I dont see a way to do that through UI, also the info gets lost when I say generate schema

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[albingroen]

Would be nice! I, unfortunately, don't have a lot of time to work on this right now though - so you're very welcome to help out :)