[Nexthop][fboss2-dev] Add a "fboss2 config interface <name> mtu <N>" command.

[benoit-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

This command allows setting the MTU on the given interface.

Note: this change is part of a series, the previous one is #760, the next one is #783.

Test Plan

Unit tests. New end to end tests for this command are in #783.

Sample usage

[admin@fboss101 benoit]$ ./fboss2-dev show interface | head
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| Interface | Status | Speed | VLAN | MTU  | Addresses                    | Description                |
--------------------------------------------------------------------------------------------------------
| eth1/1/1  | down   | 800G  | 2001 | 1500 | 10.0.0.0/24                  | Test port for diff command |
|           |        |       |      |      | 2400::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| eth1/2/1  | down   | 200G  | 2003 | 9216 | 11.0.0.0/24                  | Another test description   |
|           |        |       |      |      | 2401::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |
[admin@fboss101 benoit]$ ./fboss2-dev config interface eth1/1/1 mtu 9000
Successfully set MTU for interface(s) eth1/1/1 to 9000
[admin@fboss101 benoit]$ ./fboss2-dev config session diff
--- /etc/coop/agent.conf	2025-11-05 12:49:14.497415902 -0800
+++ /home/admin/.fboss2/agent.conf	2025-11-05 12:57:27.123758309 -0800
@@ -369,7 +369,7 @@
         ],
         "isStateSyncDisabled": true,
         "isVirtual": false,
-        "mtu": 1500,
+        "mtu": 9000,
         "routerID": 0,
         "scope": 0,
         "type": 1,
[admin@fboss101 benoit]$ ./fboss2-dev config session commit
Config session committed successfully and config reloaded.
[admin@fboss101 benoit]$ ./fboss2-dev show interface | head
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| Interface | Status | Speed | VLAN | MTU  | Addresses                    | Description                |
--------------------------------------------------------------------------------------------------------
| eth1/1/1  | down   | 800G  | 2001 | 9000 | 10.0.0.0/24                  | Test port for diff command |
|           |        |       |      |      | 2400::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |
+-----------+--------+-------+------+------+------------------------------+----------------------------+
| eth1/2/1  | down   | 200G  | 2003 | 9216 | 11.0.0.0/24                  | Another test description   |
|           |        |       |      |      | 2401::/64                    |                            |
|           |        |       |      |      | fe80::b4db:91ff:fe95:ff07/64 |                            |