apstra_freeform_link resource: detect endpoint type: internal vs. external

[chrismarget-j]

The apstra_freeform_link resource is currently unaware of the type (internal/external) of the system specified by the user.

External systems must not have interface_name and transformation_id specified (API enforces this).

Internal systems should not have interface_name and transformation_id specified, but the API accepts a half-baked endpoint specification.

If we knew the type (internal/external) of the generic systems, we could improve things by:

• not allowing half-baked links to be created
• replacing the API error with our own error

Consider:

• create a map[string]bool indicating whether each system is internal during Create().
• store the map in provider private data
• error during Create() and Update() if the user over or under specifies the link.

This discussion started in #823