storcon: Make node deletion process cancellable (#12320)

## Problem

The current deletion operation is synchronous and blocking, which is
unsuitable for potentially long-running tasks like. In such cases, the
standard HTTP request-response pattern is not a good fit.

## Summary of Changes

- Added new `storcon_cli` commands: `NodeStartDelete` and
`NodeCancelDelete` to initiate and cancel deletion asynchronously.
- Added corresponding `storcon` HTTP handlers to support the new
start/cancel deletion flow.
- Introduced a new type of background operation: `Delete`, to track and
manage the deletion process outside the request lifecycle.

---------

Co-authored-by: Aleksandr Sarantsev <aleksandr.sarantsev@databricks.com>

storage_controller/src/node.rs

@@ -201,6 +201,7 @@ impl Node {
 
         match self.scheduling {
             NodeSchedulingPolicy::Active => MaySchedule::Yes(utilization),
+            NodeSchedulingPolicy::Deleting => MaySchedule::No,
             NodeSchedulingPolicy::Draining => MaySchedule::No,
             NodeSchedulingPolicy::Filling => MaySchedule::Yes(utilization),
             NodeSchedulingPolicy::Pause => MaySchedule::No,