Provide better reverse access to classes under `org.springframework.shell.support.table`

The respected classes under org.springframework.shell.support.table are not only use solely for formatting tabular console output but are used for testing purposes as well (e.g. to introspect return shell-comment return values).

Therefore:

• Make sure that values can be retried based on the table header name as a key. This will help to minimize brittleness of test-cases where right now we have to access values using numeric values (Column number)
• add toString() to all classes