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.

libs/compute_api/src/spec.rs

@@ -67,6 +67,15 @@ pub struct ComputeSpec {
     #[serde(default)]
     pub disk_quota_bytes: Option<u64>,
 
+    /// Disables the vm-monitor behavior that resizes LFC on upscale/downscale, instead relying on
+    /// the initial size of LFC.
+    ///
+    /// This is intended for use when the LFC size is being overridden from the default but
+    /// autoscaling is still enabled, and we don't want the vm-monitor to interfere with the custom
+    /// LFC sizing.
+    #[serde(default)]
+    pub disable_lfc_resizing: Option<bool>,
+
     /// Expected cluster state at the end of transition process.
     pub cluster: Cluster,
     pub delta_operations: Option<Vec<DeltaOp>>,