[META] Add changed timestamp tracking to content entities

Problem/Motivation

Many of the core content entities (eg. menu items, custom blocks, etc) don't have changed timestamp tracking. This is a problem for various things.

1. Data migration / content push would need to check uuids and last changed timestamps to optimise content pushing.
2. Editing conflict / race condition tracking where multiple people attempt to edit and save on top of each other also needs change timestamp tracking.
3. Cache invalidation should use last change timestamps on entities, such as the menu cache based on menu link item entities. We need this data.

Proposed resolution

1. Add an interface for entities that support changed timestamps: #2044583: Add EntityChangedInterface to allow entities with "changed" field to be properly cached
2. Expand on changed timestamp support to more entities:
   • #2074193: Add changed time tracking to custom blocks
   • #2074203: Add changed time tracking to menu links
   • #2074229: Add changed time tracking to taxonomy terms
   • #2074255: Add changed time tracking to users
   • #2074475: Rename {file_managed}.timestamp to 'changed' and add a created timestamp
3. Generalize node changed validation: #2082509: Generalize node changed constraint to entity changed constraint

User interface changes

None.

API changes

Add interface for change tracking. Expand use of that, change schemas to introduce change timestamps.

Related issues

#2078387: Add an EntityAuthorInterface