Foundation for v2 serialization and deserialization (#23204)

This supports serialization of only Collections and Settings so far, but
it demonstrates the design of the new serialization system.

`metabase.models.serialization.base` defines the multimethods, which
are to be implemented by all the exported models eventually.
The actual serialization code that drives the larger process is in
`metabase_enterprise.serialization.v2.extract` and `.merge`, since
serialization is an enterprise feature.

The design calls for two matching phases on each side:
- Serialization is extract + store;
- Deserialization is ingest + load.

Extract and load deal with vanilla Clojure maps with a `serdes/meta` key
giving common details; they deliberately know nothing about files.

Store and ingest deal with the storage medium and the process of
listing and reading a stored export.

Laziness is retained: the `load` process ingests full details on demand,
so only the metadata of the importing database needs to fit in memory.