Add ability to read computed runtime values of IO subsystem buffer-pool attributes

In IO subsystem there are some attributes which are calculated based on available system resources if not explicitly specified. These attributes are:

• worker
  • io-threads
  • task-max-threads
• buffer-pool
  • buffer-size
  • buffers-per-slice
  • direct-buffers

Currently these computed values are not visible for user in the subsystem configuration even with include-runtime=true.
To show these runtime values would definitely improve UX.

Worker attributes are covered by EAP7-616 .
This issue is about buffer-pool attributes.