Unify the IPPool subnet and ExteranalIPPool subnet definition

[luolanzone]

There are two CRDs in Antrea to define an IP Pool including ippools.crd.antrea.io and externalippools.crd.antrea.io. In IPPool definition, there is a IPPoolSpec defined, this old definition can be refined to use the same way as ExternalIPPool to define IP ranges and subnet to be consistent since they actually have the same purpose.

The version of IPPool CRD may be upgraded to v1beta1 with this change. @jianjuns do you have any comment for the version change? Thanks.

[tnqn]

#5799 (comment) has some context. The goal is to make IPPoolSpec of IPPool use the same way to define IP ranges and subnet as ExternalIPPool to unify the usage and for simplicity in its beta version.

type IPPoolSpec struct {
	// IP Version for this IP pool - either 4 or 6
	IPVersion IPVersion `json:"ipVersion"`
	// The IP ranges of this IP pool, e.g. 10.10.0.0/24, 10.10.10.2-10.10.10.20, 10.10.10.30-10.10.10.30.
	IPRanges []IPRange `json:"ipRanges"`
	// The Subnet info of this IP pool. If set, all IP ranges in the IP pool should share the same subnet attributes.
	SubnetInfo *SubnetInfo `json:"subnetInfo,omitempty"`
}

I'm not sure if we still want to have IPVersion field because it can be got from the IP range and the subnet info. If it's for validation only, it may be not needed as we can just validate all IP ranges are of the same version and match the subnet.

@jianjuns may have more comments.