Locking field.storage should not lock field instance settings (field.field)

Problem/Motivation

TL;DR:
locked: true in field.storage is a good thing, but needs to be a lot more granular. Specifically, it should not disallow editing the field instance defaults. Field storage lock should only lock storage related settings and prevent deleting the field storage. Field instance should have their own protection.

Of course there are cases, where both is needed in combination, but we had several cases, where we needed this to be more granular. Especially to be able to still change field instance default settings! (mostly for paragraphs, where fields were reused several times).

Steps to reproduce

Set locked: true on a field storage
Try to adjust the field instance default settings. This is not possible in UI, while it has nothing to do with the field storage, but is field instance configuration!

Proposed resolution

There are several possible solutions:
a) Allow editing the field instance related settings (from field.field.X....yml) if the field.storage is locked and only disallow changing the storage settings
b) Move the field default settings over from the field settings into the "Manage form settings" to allow per-form-display defaults

PLUS:

• Optionally implement locked: true for field instance configurations
• or even more flexible split locked into editable and deletable for storage and instance, as suggested here: #1297278: Replace field.locked by field.editable, field.deleteable, fieldinstance.editable, fieldinstance.deleteable

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet