Finalize API for creating, overriding, and altering code-defined bundle fields

Problem/Motivation

• ContentEntityInterface::bundleFieldDefinitions() and hook_entity_bundle_field_info() can return either bundle-specific overrides of base field definitions, or definitions of fields that are not base fields. In this issue, let's call the latter case "bundle fields".
• HEAD's current example of hook_entity_bundle_field_info() creates a bundle field via BaseFieldDefinition::create(), but that is incorrect, because BaseFieldDefinition::isBaseField() returns TRUE, which is incorrect for a bundle field.
• Core's only real implementation of bundle fields is via field_entity_bundle_field_info(), which implements them via config entities.
• So, in theory, contrib modules can implement non-config-based bundle fields, but we don't provide an implementation class with which to do so.
• Additionally, we have 'base_field_override' config entities for using config to override a code-defined base field, but do not have 'bundle_field_override' config entities for overriding code-defined bundle fields. So a contrib module attempting to implement a bundle field implementation would also need to implement a getConfig() method so that content_translation and contrib extensions like field_permissions could work on them.
• And hook_entity_bundle_field_info_alter() has the same problem as 🐛 hook_entity_base_field_info_alter() and hook_entity_bundle_field_info_alter() are documented to get a parameter that doesn't implement an interface with setter methods Needs work .

Proposed resolution

Remaining tasks

Figure out how to fix all of the above.

User interface changes

API changes