feat: declare get_management_peer for choosing an SNMP agent's sources

The caller has to name the sources an SNMP agent answers, and cannot
reliably tell its own address as the device sees it (NAT, relays).
SnmpAgentMixin now declares get_management_peer(): the address the
device sees the management connection come from, or None. Optional:
a driver that cannot tell does not implement it.

napalm_device_types/fixes.py

@@ -22,7 +22,7 @@ They replace ``run_device_action`` (docs/STRUCTURE.md):
 
 from __future__ import annotations
 
-from typing import Any, Callable, List, Sequence, Tuple, TYPE_CHECKING
+from typing import Any, Callable, List, Optional, Sequence, Tuple, TYPE_CHECKING
 
 from napalm_device_types.models import (
     FixResultDict,
@@ -77,7 +77,16 @@ class SnmpAgentMixin:
             ]
         )
 
-    if TYPE_CHECKING:  # pragma: no cover - the steps a driver implements
+    if TYPE_CHECKING:  # pragma: no cover - declared for type checkers only
+
+        def get_management_peer(self) -> Optional[str]:
+            """The address the device sees this management connection come from
+            (for SSH, the client side of ``SSH_CONNECTION``), or None when it cannot
+            tell. A fact for the caller choosing ``allowed_sources``; optional --
+            a driver that cannot tell does not implement it."""
+            ...
+
+        # The steps a driver implements:
 
         def _install_snmp_agent(self) -> FixStepOutcomeDict: ...