compute: Add spec support for disabling LFC resizing (#10132)

ref neondatabase/cloud#21731

## Problem

When we manually override the LFC size for particular computes,
autoscaling will typically undo that because vm-monitor will resize LFC
itself.

So, we'd like a way to make vm-monitor not set LFC size — this actually
already exists, if we just don't give vm-monitor a postgres connection
string.

## Summary of changes

Add a new field to the compute spec, `disable_lfc_resizing`. When set to
`true`, we pass in `None` for its postgres connection string. That
matches the configuration tested in `neondatabase/autoscaling` CI.

control_plane/src/endpoint.rs

@@ -585,6 +585,7 @@ impl Endpoint {
             features: self.features.clone(),
             swap_size_bytes: None,
             disk_quota_bytes: None,
+            disable_lfc_resizing: None,
             cluster: Cluster {
                 cluster_id: None, // project ID: not used
                 name: None,       // project name: not used