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/schema.prisma

@@ -31,9 +31,11 @@ model Experiment {
 model PromptVariant {
     id String @id @default(uuid()) @db.Uuid
 
-    label       String
-    constructFn String
-    model       String
+    label              String
+    constructFn        String
+    constructFnVersion Int
+    model              String
+    modelProvider      String
 
     uiId      String  @default(uuid()) @db.Uuid
     visible   Boolean @default(true)
@@ -98,6 +100,7 @@ model ScenarioVariantCell {
 
     promptVariantId String        @db.Uuid
     promptVariant   PromptVariant @relation(fields: [promptVariantId], references: [id], onDelete: Cascade)
+    prompt          Json?
 
     testScenarioId String       @db.Uuid
     testScenario   TestScenario @relation(fields: [testScenarioId], references: [id], onDelete: Cascade)