Expose APIs to configure underlying network priority

This commit expose VcnUnderlyingNetworkTemplate classes
and methods to configure those objects in VcnGatewayConnectionConfig.

Those APIs allow callers to configure the underlying network priority
for VCN gateway when VCN is doing route selection. Callers can
construct VcnUnderlyingNeworkTemplate with network requirements and
then set VcnUnderlyingNeworkTemplate list from most preffered to least
preffered in VcnGatewayConnectionConfig.

Bug: 205343355
CTS-Coverage-Bug: 209498272
Test: atest FrameworksVcnTests, CtsVcnTestCases
Test: make update-api
Change-Id: Ib7d51cf1ce562a1acbcb767ec569d7e11c861a1d