[enrich] Fix enrich items with multiple enrollments

[zhquan]

This commit fixes the date format when there are several enrollments in profiles. Since the number of times the str_to_datetime method is executed has been reduced to improve performance the date field in get_item_sh_from_id and get_item_sh_meta_fields must be strings.

Test to enrich a profile with multiple enrollments added.

[coveralls]

Pull Request Test Coverage Report for Build 5622670110

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 62 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 82.973%

---

Files with Coverage Reduction	New Missed Lines	%
/home/runner/work/grimoirelab-elk/grimoirelab-elk/grimoire_elk/enriched/enrich.py	62	75.72%
<!--	Total:	62		-->

Totals
Change from base Build 5574616499:	0.05%
Covered Lines:	9371
Relevant Lines:	11294

---

💛 - Coveralls