fix: add static route for custom vpc when create subnet with U2OInterconnection.

[xujunjie-cover]

[U2OInterconnection.](fix: add static route for custom vpc when create subnet with U2OInterconnection.)

Pull Request

• Make sure you have followed Kube-OVN Code Style.

What type of this PR

Examples of user facing changes:

• Bug fixes

Which issue(s) this PR fixes

Fixes #3446

WHAT

🤖[deprecated] Generated by Copilot at d8a6584

This pull request enhances the subnet controller to support multiple VPCs and static routes. It allows subnets in different VPCs to communicate with each other through the underlay network. It updates the subnet.go file with new functions and logic for managing the static route operations.

🤖[deprecated] Generated by Copilot at d8a6584

We're sailing on the cloud with multiple VPCs
We need to interconnect them with static routes
So haul away the subnet controller and make it work
One, two, three, heave ho, and update the underlay

HOW

🤖[deprecated] Generated by Copilot at d8a6584

• Add or delete static routes for underlay to overlay subnet interconnection when creating, updating, or deleting subnets (link, link, link)
• Define helper functions addStaticRouteForU2OInterconn and deleteStaticRouteForU2OInterconn in pkg/controller/subnet.go (link)

[oilbeater]

@xujunjie-cover please fix the lint issue