feat: convert virtual circuit resource & datasource to equinix-sdk-go

[ctreatma]

This updates the metal_virtual_circuit resource and data source to use equinix-sdk-go instead of packngo. As mentioned in #402, the packngo SDK has been deprecated, and moving to equinix-sdk-go makes it possible for these resources to gain new features in the future.

As an added bonus, the affected code has also been moved to internal/resources/metal to align with #106.

[displague]

Noting some failed sweeps:

2024/06/18 22:09:56 Sweeper Tests for region (all) ran unsuccessfully:
2024/06/18 22:09:56 	- equinix_metal_vrf: Error deleting VRFs DELETE https://api.equinix.com/metal/v1/vrfs/97b74582-92ab-4df3-af96-a2efe41e3e97: 422 Cannot delete VRF with associated Virtual Circuits, please delete any associated virtual circuits first 
2024/06/18 22:09:56 	- equinix_metal_project: 2 errors occurred:
	* Error deleting project DELETE https://api.equinix.com/metal/v1/projects/47b9d47f-57fd-4e3d-aa0c-d0b812bd18ce: 422 Cannot delete a project with active virtual circuits. 
	* Error deleting project DELETE https://api.equinix.com/metal/v1/projects/d3[135](https://github.com/equinix/terraform-provider-equinix/actions/runs/9572657239/job/26392474481?pr=700#step:7:136)1a8-fb6f-4b76-b835-4453971adec7: 422 Cannot delete a project with active virtual circuits. 


2024/06/18 22:09:56 	- equinix_metal_virtual_circuit: error getting organization list for sweeping VirtualCircuits: no value given for required property address

[displague]

Some sweeper errors persist:

2024/06/19 16:46:55 Sweeper Tests for region (all) ran unsuccessfully:
2024/06/19 16:46:55 	- equinix_metal_project: 1 error occurred:
	* Error deleting project DELETE https://api.equinix.com/metal/v1/projects/f096688e-166d-42f1-9959-9acf97ebc8a1: 422 cannot delete when associated to a virtual circuit 


2024/06/19 16:46:55 	- equinix_metal_vlan: Error deleting vlan DELETE https://api.equinix.com/metal/v1/virtual-networks/851f2fc7-bfc1-4a9b-b8a7-4a8ec4e125f1: 422 Cannot delete Virtual Network when associated to a virtual circuit 
FAIL	github.com/equinix/terraform-provider-equinix/internal/sweep	7.564s
FAIL

[displague]

I want to think there is some pattern to be used across this resource that would have collapsed the if vrf {} else vlan {} blocks in this PR.

That could look like this:

type VCCommon interface {
  GetHref() string
  GetID() string
  Set...
}

Where that quickly falls apart is in the enum inputs/outputs and other one-off differences between VirtualCircuit types, such as int reference vs *int for NNID fields.

Within resourceMetalVirtualCircuitUpdate, it is more obvious to see how we could apply that pattern. We did so in metal-cli.

equinix/metal-cli#369 (comment)

I only mention this for posterity. Not looking for this PR to adopt this pattern.

[ctreatma]

The remaining sweeper errors seem to pre-date this PR: https://github.com/equinix/terraform-provider-equinix/actions/runs/9503755717/job/26194913301#step:7:146

The connection in question does not have any virtual circuits associated, so it appears they were deleted, just not in time for the subsequent sweeper. That's a problem that impacts our sweepers in general: at a glance, approximately none of them wait for swept resources to disappear; they just assume that if the delete request passed, all is well.