Bug 33749: Add TrimFields field class

Bug 33749: Add TrimFields field class

This patch adds a RecordProcessor filter class that takes care of
performing the field cleanup that was originally done in the static
method `Koha::MetadataRecord::stripWhitespaceChars`.

In order to test the results, I adapted the tests to use the filter
instead of the original method.

To test:
1. Run:
   $ ktd --shell
  k$ prove t/Koha_MetadataRecord.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests still pass
4. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>