Support any content entity type, not only nodes and taxonomy terms

Problem/Motivation

A module that claims to expose the content model of any site cannot be limited to nodes and terms. On one of the sites this module runs on the products are commerce entities rather than nodes; other sites keep content in media, in profiles, in entity types of their own. The current base classes and helpers assume a node or a term in signatures, in loading, in alias handling and in language handling.

Proposed resolution

• Type against ContentEntityInterface and the entity type manager throughout, rather than against node and term classes.
• Take the bundle key, the label key and the alias source from the entity type definition instead of assuming type/title//node/{nid}.
• Handle entity types that have no bundles and ones that have no canonical link template — both exist and both currently produce fatals or empty aliases.
• Kernel coverage over at least four entity types: node, taxonomy term, media, and a custom one from the test module.

Remaining tasks

Everything. Blocks the scanner, which otherwise learns to scan only what the resources can serve.