The stack type for this VPN gateway to identify the IP protocols
that are enabled. Possible values are: IPV4_ONLY, IPV4_IPV6. If not
specified, IPV4_ONLY will be used.
Values:
UNDEFINED_STACK_TYPE (0):
A value indicating that the enum field is not
set.
IPV4_IPV6 (22197249):
Enable VPN gateway with both IPv4 and IPv6
protocols.
IPV4_ONLY (22373798):
Enable VPN gateway with only IPv4 protocol.