Add support for Metal Gateway BGP Dynamic Neighbors

[displague]

What problem are you facing?

For complete Metal Gateway support (#205) there are a few new features of Metal Gateway yet to be implemented. Among them is BGP Dynamic Neighbors. This also ties to VRF (#268) as the features are closely associated.

https://github.com/equinix-labs/metal-go/blob/main/docs/VRFsApi.md

Method	HTTP request	Description
BgpDynamicNeighborsIdGet	Get /bgp-dynamic-neighbors/{id}	Retrieve a BGP Dynamic Neighbor
CreateBgpDynamicNeighbor	Post /metal-gateways/{id}/bgp-dynamic-neighbors	Create a VRF BGP Dynamic Neighbor range
DeleteBgpDynamicNeighborById	Delete /bgp-dynamic-neighbors/{id}	Delete a VRF BGP Dynamic Neighbor
GetBgpDynamicNeighbors	Get /metal-gateways/{id}/bgp-dynamic-neighbors	List BGP Dynamic Neighbors

How could the Equinix Metal CLI help solve this problem?

metal gateway list-bgp-dynamic-neighbors --id <gateway_id>
metal gateway create-bgp-dynamic-neighbors --id <gateway_id>
metal gateway delete-bgp-dynamic-neighbors --bgp_dynamic_neighbor_id <bgp_dynamic_neighbor_id>
metal gateway get-bgp-dynamic-neighbors --bgp_dynamic_neighbor_id <bgp_dynamic_neighbor_id>

These command names are wordy, open to suggestions.

[aayushrangwala]

/assign @aayushrangwala