Prep for more model providers

Adds a `modelProvider` field to `promptVariants`, currently just set to "openai/ChatCompletion" for all variants for now.

Adds a `modelProviders/` directory where we can define and store pluggable model providers. Currently just OpenAI. Not everything is pluggable yet -- notably the code to actually generate completions hasn't been migrated to this setup yet.

Does a lot of work to get the types working. Prompts are now defined with a function `definePrompt(modelProvider, config)` instead of `prompt = config`. Added a script to migrate old prompt definitions.

This is still partial work, but the diff is large enough that I want to get it in. I don't think anything is broken but I haven't tested thoroughly.

prisma/migrations/20230720213215_add_model_providers/migration.sql

@@ -0,0 +1,17 @@
+-- Add new columns allowing NULL values
+ALTER TABLE "PromptVariant" 
+ADD COLUMN "constructFnVersion" INTEGER, 
+ADD COLUMN "modelProvider" TEXT;
+
+-- Update existing records to have the default values
+UPDATE "PromptVariant" 
+SET "constructFnVersion" = 1, 
+"modelProvider" = 'openai/ChatCompletion' 
+WHERE "constructFnVersion" IS NULL OR "modelProvider" IS NULL;
+
+-- Alter table to set NOT NULL constraint
+ALTER TABLE "PromptVariant" 
+ALTER COLUMN "constructFnVersion" SET NOT NULL, 
+ALTER COLUMN "modelProvider" SET NOT NULL;
+
+ALTER TABLE "ScenarioVariantCell" ADD COLUMN     "prompt" JSONB;