Field method refactor for Ead transform

Why these changes are being introduced:
* These updates are required to implement the architecture described
in the following ADR: https://github.com/MITLibraries/transmogrifier/blob/main/docs/adrs/0005-field-methods.md

How this addresses that need:
* Rename param 'xml' to 'source_record'
* Add private methods for retrieving key elements:
  <archdesc>, <archdesc><did>, <controlaccess>
* Raise SkippedRecordEvent when <archdesc> and <archdesc><did>
  are missing
* Add field methods for alternate_titles, citation, content_type
* Use named args for 'create_list/string_from_mixed_value'
* Update tests
  * Rename tests
  * Add method to create source records from EAD-formatted XML for unit tests
  * Add tests for field methods alternate_titles, main_titles, citation, content_type
  * Avoid linting error E501

Side effects of this change:
* None

Relevant ticket(s):
* https://mitlibraries.atlassian.net/browse/TIMX-287