feat(api): Implement custom provider API integration

- Add CustomHandler for handling custom provider API requests
- Add custom provider types and interfaces in shared/api.ts
- Support streaming and non-streaming responses
- Add token usage tracking and path-based response parsing
- Add custom provider to buildApiHandler factory
- Update API interfaces to support custom provider options

Author: Oct4Pie

src/api/index.ts

@@ -15,6 +15,7 @@ import { MistralHandler } from "./providers/mistral"
 import { VsCodeLmHandler } from "./providers/vscode-lm"
 import { ApiStream } from "./transform/stream"
 import { UnboundHandler } from "./providers/unbound"
+import { CustomHandler } from "./providers/custom"
 
 export interface SingleCompletionHandler {
 	completePrompt(prompt: string): Promise<string>
@@ -56,6 +57,8 @@ export function buildApiHandler(configuration: ApiConfiguration): ApiHandler {
 			return new MistralHandler(options)
 		case "unbound":
 			return new UnboundHandler(options)
+		case "custom":
+			return new CustomHandler(options) as unknown as ApiHandler // Type assertion since CustomHandler is compatible through transformation
 		default:
 			return new AnthropicHandler(options)
 	}