Merged PR 320: #2419- Created a Router-specific version of SessionManager

## Summary
Created a Router-specific version of `SessionManager` that looks at route table rather than default gateway when dst ip address isn't for a locally attached network. Carried these changes through to arp. Added test for this. Made some minor improvements to show functions in container and node that assist debugging.

## Test process
TDD approach: wrote the test, ran it and watched it fail. The implemented the fix and re-ran the test; it passes.

**Before fix:**
![image (2).png](https://dev.azure.com/ma-dev-uk/b50a61ee-86c4-48bc-9a0b-a67645ba12ee/_apis/git/repositories/2825053e-bd3b-45b2-8680-1281809eefa2/pullRequests/320/attachments/image%20%282%29.png)

**After fix:**
![image.png](https://dev.azure.com/ma-dev-uk/b50a61ee-86c4-48bc-9a0b-a67645ba12ee/_apis/git/repositories/2825053e-bd3b-45b2-8680-1281809eefa2/pullRequests/320/attachments/image.png)

## Checklist
- [ ] PR is linked to a **work item**
- [ ] **acceptance criteria** of linked ticket are met
- [ ] performed **self-review** of the code
- [ ] written **tests** for any new functionality added with this PR
- [ ] updated the **documentation** if this PR changes or adds functionality
- [ ] written/updated **design docs** if this PR implements new functionality
- [ ] updated the **change log**
- [ ] ran **pre-commit** checks for code style
- [ ] attended to any **TO-DOs** left in the code

#2419

Related work items: #2419

src/primaite/simulator/system/core/session_manager.py

@@ -72,7 +72,6 @@ class SessionManager:
     Manages network sessions, including session creation, lookup, and communication with other components.
 
     :param sys_log: A reference to the system log component.
-    :param arp_cache: A reference to the ARP cache component.
     """
 
     def __init__(self, sys_log: SysLog):

src/primaite/simulator/system/services/arp/arp.py

@@ -65,6 +65,10 @@ class ARP(Service):
         """Clears the arp cache."""
         self.arp.clear()
 
+    def get_default_gateway_network_interface(self) -> Optional[NetworkInterface]:
+        """Not used at the parent ARP level. Should return None when there is no override by child class."""
+        return None
+
     def add_arp_cache_entry(
         self, ip_address: IPV4Address, mac_address: str, network_interface: NetworkInterface, override: bool = False
     ):