add auto-discovery of bgp peers using default gateway by fetching default routes from statedb

Fixes: #37315

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This PR adds a new feature called bgp auto-discovery to discover bgp peers dynamically and create bgp sessions with it.
Auto-discovery can use multiple modes to discovery the peers. This change will include default-gateway mode to discovery peers.
Proposal: https://github.com/cilium/design-cfps/blob/main/cilium/CFP-37315-bgp-auto-discovery.md

When a cilium user uses default-gateway mode for auto-discovery for a specific address family, cilium will fetch the routes from route table of statedb and link data from device table of statedb.
It will filter default routes for the given address family and check if the link through which the default route is reachable is UP or not (it will use operational status of the link from the device table for up/down). If its down, we will ignore it.
Finally, it will use the default gateway of the default route with the lowest priority.

What will happen if the default route's attributes like priority or the link status has changed?
cilium will track of route event changes using a separate go-routine. If there is any change in the link or default route, route update will be triggered and we will run bgp reconciliation to find peers again.

Steps for testing this change with multi-homing:-

1. create kind cluster
   sudo make kind-bgpv2-dg-multi-homing

2. build cilium
   sudo KIND_CLUSTER_NAME=bgpv2-cplane-dev-mh make kind-image

3. Install cilium into the kind cluster
   sudo cilium install --chart-directory install/kubernetes/cilium -f contrib/containerlab/bgpv2/auto-discovery/default-gateway/multi-homing/values.yaml --set image.override="localhost:5000/cilium/cilium-dev:local" --set image.pullPolicy=Never --set operator.image.override="localhost:5000/cilium/operator-generic:local" --set operator.image.pullPolicy=Never

4. apply bgp policy (contrib/containerlab/bgpv2/auto-discovery/default-gateway/bgp.yaml)
   sudo make kind-bgpv2-dg-multi-homing-apply-bgp

Fixes: #37315

BGP: Add peer auto-discovery using default-gateway mode