[PP-2] JSON:API relationship/related support for config entities

Problem/Motivation

Follow-up for #2300677: JSON:API POST/PATCH support for fully validatable config entities.

"relationships" and "related" support was descoped from that, because:

• Relationships between config entities (for example NodeType may have associated FieldConfig and FieldConfigneeds FieldStorageConfig) typically require hardcoded logic in the corresponding client/UI anyway. So expressing the relationships through links/hypermedia is not blocking such UIs from being built.
• The explicit required config dependencies (such as FieldConfig needs a FieldStorageConfig) are very clearly expressed (thanks to RequiredConfigDependencies, since #3324150: Add validation constraints to config_entity.dependencies). We could autogenerate a sensible JSON:API relationship name for this, in both directions. But … other relationships are less explicit. For example, FieldConfig is associated with a particular content entity type + bundle. For nodes, that'd be a NodeType. For example, the body field in the Standard install profile contains this:
  

  dependencies:
    config:
      - field.storage.node.body
      - node.type.article
    module:
      - text
  

  … the field.storage.node.body could be described by RequiredConfigDependencies, but what about node.type.article? How do we generate a sensible name for it?

Steps to reproduce

N/A

Proposed resolution

TBD

Remaining tasks

Discuss, propose!

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD