Support parallel tool calls on client

[rclmenezes]

Feature Description

Currently, processChatResponse's onToolCallPart is blocking:

https://github.com/vercel/ai/blob/main/packages/ui-utils/src/process-chat-response.ts#L291-L3](https://github.com/vercel/ai/blob/main/packages/ui-utils/src/process-chat-response.ts#L291-L313)

There's a comment saying "In the future we should make this non-blocking, which requires additional state management for error handling etc."

Use Cases

Parallel tool uses in the client! I'm making a chrome extension that uses Vercel AI and this would be very helpful

Additional context

I'm not entirely sure how to implement this. Maybe we append each tool use to a list of promises in processChatResponse and return once they're all settled?

We could introduce some p-all like functionality to control amount of concurrency as well