Allow creating instances without waiting

[jvstme]

Problem

InstancesService.create blocks for up to 180 seconds by default while waiting for an instance to enter the provisioning state. While the timeout can be adjusted, the wait cannot be disabled. This behavior causes the following issues in dstack:

1. Instance loss
   We have encountered multiple cases where our application lost track of an instance due to an exception raised during the wait. This can happen, for example, when:

   • the instance takes longer than 180 seconds to enter the provisioning state, or
   • a network error occurs during one of the get_by_id calls.

   Because these exceptions are raised before InstancesService.create returns the instance ID, our application has no opportunity to persist the ID. As a result, the instance continues running and accruing charges, but is no longer tracked by the application.

2. Performance
   Our application architecture is optimized for short-lived transactions. Blocking for up to 180 seconds can lead to performance issues, such as database connection pool exhaustion.

Solution

Allow instance creation without blocking.

Ideally, the instance creation method should return immediately once the instance ID is known, without performing any subsequent API calls. This would allow us to persist the instance ID reliably and manage instance state polling using our own implementation.