Bug 30504: Value builder for field 181

Bug 30504: Value builder for field 181

This patch provides values builders for the subfields $a, $b, $c and $2. Those are unimarc official codes who help to identify the kind of items

Test plan:
1)Go to Home > Cataloging > New record
2)Section 1 - field 181 : it contains subfields $a, $b, $c & $2
3)Inputs are writable but notice that no plugins are suggested
4)Apply patch and repeat 1) to 3)
5)Values builders plugins are available for $a, $b, $c and $2

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>