[WAITING FOR SPEC UPDATE] Add award fields signingPeriod, admissionPeriod

signingPeriod, admissionPeriod - related to award entity. Proofs: https://confluence-sale.prozorro.org/pages/viewpage.action?pageId=50365887, https://confluence-sale.prozorro.org/pages/viewpage.action?pageId=47448432 (look for *, *** marks)

Wait for fields spec update on table: https://docs.google.com/spreadsheets/d/1P3PDY0RPXewHPat0CTSXW_K3bq29bJDefCgzFWBDgAU/edit?ts=5dc421bd&pli=1#gid=558852150