storcon: Introduce deletion tombstones to support flaky node scenario (#12096)

## Problem

Removed nodes can re-add themselves on restart if not properly
tombstoned. We need a mechanism (e.g. soft-delete flag) to prevent this,
especially in cases where the node is unreachable.

More details there: #12036

## Summary of changes

- Introduced `NodeLifecycle` enum to represent node lifecycle states.
- Added a string representation of `NodeLifecycle` to the `nodes` table.
- Implemented node removal using a tombstone mechanism.
- Introduced `/debug/v1/tombstone*` handlers to manage the tombstone
state.

test_runner/fixtures/neon_fixtures.py

@@ -2054,6 +2054,14 @@ class NeonStorageController(MetricsGetter, LogUtils):
             headers=self.headers(TokenScope.ADMIN),
         )
 
+    def tombstone_delete(self, node_id):
+        log.info(f"tombstone_delete({node_id})")
+        self.request(
+            "DELETE",
+            f"{self.api}/debug/v1/tombstone/{node_id}",
+            headers=self.headers(TokenScope.ADMIN),
+        )
+
     def node_drain(self, node_id):
         log.info(f"node_drain({node_id})")
         self.request(
@@ -2110,6 +2118,14 @@ class NeonStorageController(MetricsGetter, LogUtils):
         )
         return response.json()
 
+    def tombstone_list(self):
+        response = self.request(
+            "GET",
+            f"{self.api}/debug/v1/tombstone",
+            headers=self.headers(TokenScope.ADMIN),
+        )
+        return response.json()
+
     def tenant_shard_dump(self):
         """
         Debug listing API: dumps the internal map of tenant shards