MF Screens - load logical unit pool from external json file

If a user would like to supply the logical units from an external source.

It is possible to enable pool via the following properties:

ol:
  h3270:
    project:
      [orch-key]:
        logical-units:
          use-pool: true
          class-name: org.openlegacy.providers.h3270.lu.ExternalJsonLogicalUnitBuilder

use-pool - default false
class-name - default org.h3270.logicalunit.SimpleLogicalUnitBuilder

Built it implementation org.openlegacy.providers.h3270.lu.ExternalJsonLogicalUnitBuilder expects a json file named logical-units.json in src/main/resources, e.g.

{
  "pool": [
    "a",
    "b",
    "c"
  ]
}