Fabric service tokens in metal_connection

[t0mk]

This PR adds Fabric service token support to metal_connection resource and datasource.

I also changed attribute validation in metal_connection, according to my understanding of what has changed (in resourceMetalConnectionCreate)

There are 2 things to do before this can be merged:

• This PR is blocked by https://github.com/packethost/packngo/issues/317, because right now it's not possible to read project ID for a created connection. Once the project attribute is added to Connection resoruce in EM API, it should be added to packngo, and then the read of project_id in resourceMetalConnectionRead in metal/resource_metal_connection.go should be updated.

• Update go mod dependency when the fabric service token support is released in packngo.

[t0mk]

This is blocked by https://github.com/packethost/packngo/issues/317