Add protected saveNew() and saveUpdated() methods to Field and FieldInstance to improve code readability

Problem/Motivation

• This is a followup for #1735118: Convert Field API to CMI and postponed on that issue. Field::save() and FieldInstance::save() include long if/else statements that execute different logic based on whether a field or instance is new or updated.

Proposed resolution

• Factor this logic out into two methods.

Remaining tasks

• Create a patch that refactors save() in this way.
• Propose how the hooks should be invoked with the refactor.