Fix warnings for server-side computed default values, by setting attributes as...

Fix warnings for server-side computed default values, by setting attributes as Computed for optional attributes.

Warning produced:
2023-02-26T07:32:21.945Z [WARN]  Provider "github.com/matthewjohn/jmon" produced an invalid plan for jmon_check.basic_check, but we are tolerating it because it is using the legacy plugin SDK.
    The following problems may be the cause of any confusing errors from downstream operations:
      - .environment: planned value cty.StringVal("default") for a non-computed attribute

Issue #1