Add support for NSX-T Edge Gateway External Uplink configuration

[Didainius]

This PR adds missing pieces to support NSX-T Edge Gateway External Network uplinks (only for external networks backed by NSX-T Segments) (VCD 10.4.1+)

List of things added:

• New internal function *NsxtEdgeGateway.reorderUplinks() that will order NSX-T Edge Gateway uplinks. There are two major types of uplinks
  • NSXT_TIER0 or NSXT_VRF_TIER0 - Tier 0 gateway backed external network (mandatory and is required during creation)
  • IMPORTED_T_LOGICAL_SWITCH NSX-T segment backed external network (this is optional, attached post creation in "External networks" tab)
• New method NsxtEdgeGateway.GetAllocatedIpCountByUplinkType complementing existing
  NsxtEdgeGateway.GetAllocatedIpCount. It will return allocated IP counts by uplink types (requires VCD 10.4.1+)
• New method NsxtEdgeGateway.GetPrimaryNetworkAllocatedIpCount that will return total allocated IP
  count for primary uplink (T0 or T0 VRF)
• New field types.EdgeGatewayUplinks.BackingType that defines backing type of NSX-T Edge Gateway
  Uplink (available in VCD 10.4.1+)

Note on uplink ordering. API usually returns the mandatory network as element 0 in the resulting slice, but there have been cases where VCD sometimes shuffles these responses (don't have a proof for this particular endpoint). As a result - the reorderUplinks function is here to ensure that NSXT_TIER0 or NSXT_VRF_TIER0uplink is always at position 0. It is also used in all functions related to NSX-T Edge Gateway.

Tested with tags nsxt gateway network:

• 10.5.0
• 10.4.1
• 10.4.0
• 10.4.2

Tests are for admin mode only, skip directive for Org tests is in place. The test requires 10.4.1+.

[lvirbalas]

Thanks!