feat: Add/Rework slurmctld statistics (#370)

* add slurmctld stats

* remove scripts/slurm_msg_type_dict.py

This is superseded by the fact that we now use libslurmfull and can
access the rpc_num2string function provided there.

* Backport Changes: Make it work with 23.11

(cherry picked from commit 5a2fcd5)

Author: tazend

docs/reference/slurmctld.md

@@ -6,3 +6,4 @@ title: slurmctld
     handler: python
     options:
         members: yes
+        members_order: source

pyslurm/core/slurmctld/__init__.py

@@ -5,6 +5,18 @@
    CgroupConfig,
 )
 from .enums import ShutdownMode
+from .stats import (
+    diag,
+    Statistics,
+    ScheduleExitStatistics,
+    BackfillExitStatistics,
+    RPCPending,
+    RPCUser,
+    RPCType,
+    RPCPendingStatistics,
+    RPCUserStatistics,
+    RPCTypeStatistics,
+)
 from .base import (
     PingResponse,
     ping,