Moderation state field definitions can have an incorrect target bundle

Problem/Motivation

After #3537962: EntityFieldManager::getFieldDefinitions() per-bundle caching can be expensive, moderation state field definitions may be built with an incorrect target bundle. This change exposed a latent bug in ContentModerationHooks::entityBundleFieldInfo() in which a base field definition is returned without first being cloned, allowing the target bundle to be mutated through a shared reference.

Steps to reproduce

1. Create two moderated bundles.
2. Inspect the field definitions for both bundles.
3. One of the moderation_state definitions will have the wrong target bundle.

Proposed resolution

Return a cloned base field definition in ContentModerationHooks::entityBundleFieldInfo().

Remarks

• It's more performant to fix this in each hook implementation, albeit less reliable than deep cloning $base_field_definitions in the entity field manager prior to each invocation.
• Since the entity field manager already sets the target bundle for each definition, that same step can be omitted in the content moderation module's hook implementation.
• I chose Major priority for this issue since it's a "significant [...] developer-facing bug with no workaround."