make AssetSelections serializable (#18588)

## Summary & Motivation

This will allow us to serialize the asset selections that are on
automation policy sensors, as part of the repository snapshot.

A tricky followup will be to find a way to deal with
`DbtManifestAssetSelection`, which isn't part of the core framework so
can't be rehydrated in a host process. We'll probably need to find some
way to convert it to a core asset selection type before serializing it.

## How I Tested These Changes