Eliminate apparent state churn with resource pool computed attributes

[chrismarget-j]

This PR eliminates apparent state churn related to always-null computed values in pool resources.

Consider a simple pool name change:

Before:

  # apstra_vni_pool.x will be updated in-place
  ~ resource "apstra_vni_pool" "x" {
        id              = "686e9c0b-e084-4e9a-9498-f9efa698969b"
      ~ name            = "bar" -> "foo"
      ~ ranges          = [
          - {
              - first = 5001 -> null
              - last  = 5010 -> null
            },
          + {
              + first           = 5001
              + last            = 5010
              + status          = (known after apply)
              + total           = (known after apply)
              + used            = (known after apply)
              + used_percentage = (known after apply)
            },
        ]
      + status          = (known after apply)
      + total           = (known after apply)
      + used            = (known after apply)
      + used_percentage = (known after apply)
    }

Note that most of the output is computed values which are going to be null anyway (#737, #752).

After:

  # apstra_vni_pool.x will be updated in-place
  ~ resource "apstra_vni_pool" "x" {
        id     = "686e9c0b-e084-4e9a-9498-f9efa698969b"
      ~ name   = "bar" -> "foo"
        # (1 unchanged attribute hidden)
    }

In this PR we have:

• introduction of a new plan modifier UseNullStateForUnknown which works like the framework's UseStateForUnknown, but it doesn't ignore null values found in the state -- these also get replicated to the plan.
• use of the new plan modifier on all of the pool metadata fields
• normalization of the MarkdownDescription inline documentation for these un-readable metadata fields

Tested with:

• 4.2.0
• 4.2.1
• 4.2.1.1
• 4.2.2
• 5.0.0

Closes #923