Eliminate bad assumption about logical interface node IDs

[chrismarget-j]

The apstra_datacenter_ip_link_addressing resource takes the ID of a logical link node as input (link_id).

The actual IP addresses need to be assigned to the logical interface nodes on either end of the link, and the associated API call requires those interface node IDs as input.

The original version of the apstra_datacenter_ip_link_addressing resource assumed that the logical link IDs were immutable, but that's only partly true: Changes to the CT which created the logical link may cause the link and associated interfaces to be replaced. The link ID is constructed from predictable values, so it doesn't change even if replaced. The interface IDs will be different when they are removed and recreated.

Changes in this PR:

• Computed attributes generic_interface_id and switch_interface_id have been removed.
• Logical interface IDs are now determined at runtime in each of the four CRUD phases.
• The original (pre-Create()) interface addressing strategy are now stored in private state (for restoration during Destroy()) using structs and methods in a new package private which can be extended to support similar use cases in the future.

Closes #978

[rajagopalans]

Approved after discussion.