Implement the controller for API BGPPolicy

[hongliangl]

This commit implements the controller of API BGPPolicy, designed to advertise
Service IPs, Egress IPs, and Pod IPs to BGP peers from selected Nodes.

According to the spec of BGPPolicy, the Node selector is used to select Nodes
to which a BGPPolicy is applied. Multiple BGPPolicies can be applied to the
same Node. However, only one BGPPolicy can be effective on a Node, with others
serving as alternatives. The first BGPPolicy applied to a Node will be the
effective one, and the latter ones will serve as alternatives. The effective one
may be changed in the following cases:

• The current effective BGPPolicy is updated and not applied to the Node.
• The current effective BGPPolicy is deleted.
• The antrea-agent is rebooted, and an original alternative BGPPolicy is synced
  first and becomes effective.

The BGP server instance is only started for the effective BGPPolicy on a Node.
Currently, the BGP server instance is implemented by goBGP, and more BGP
server implementations might be added later. If the effective BGPPolicy is changed,
the corresponding BGP server instance will be terminated by calling the Stop()
method, and a new BGP server instance will be created and started by calling the
Start() method for the new effective BGPPolicy.

To start a BGP server instance, ASN, routerID, and listen port must be specified.
ASN and listen port are specified in the spec of the effective BGPPolicy. For
routerID, if the cluster is IPv4 only or dual stack, the IPv4 NodeIP is used as
the routerID; if the cluster is IPv6 only, the routerID must be specified in the
Node annotation antrea.io/bgp-route-id. Additionally, a new BGP server instance
should be created and started when any of ASN, routerID, or listen port changes.

The information of the BGP peers is specified in the effective BGPPolicy. The
unique identification of a BGP peer is the peer IP address.

To reconcile the latest BGP peers:

• Get the BGP peers to be added and add them by calling the AddPeers method of
  the BGP server instance.
• Get the BGP peers to be deleted and delete them by calling the RemovePeers
  method of the BGP server instance.
• Get the remaining BGP peers and calculate the updated BGP peers, then update
  them by calling the UpdatePeers method of the BGP server instance.

The information of the IPs to be advertised can be calculated from the spec of
the effective BGPPolicy. Currently, we advertise the IPs and CIDRs to all the
BGP peers.

To reconcile the latest IPs to all BGP peers:

• If the BGP server instance is newly created and started, advertise all the IPs
  by calling the AdvertiseRoutes method.
• If the BGP server instance is not newly created and started:
  • Get the IPs/CIDRs to be added and advertise them by calling the
    AdvertiseRoutes method.
  • Get the IPs/CIDRs to be removed and withdraw them by calling the
    WithdrawRoutes method.

The feature is gated by the alpha BGPPolicy feature gate.

Depends on #6009

• goBGP integration tests
• e2e tests
• unit tests
• refine code comments
• documents
• goBGP logger