Improve asset resolver cache keys

Problem/Motivation

The cache keys we use in AssetResolver serialize the libraries array, but this is an array of strings so we can just use implode() instead, which also makes it a bit easier to see the distribution of libraries when inspecting cache bins.

Additionally, when we filter libraries by key, we don't rekey the array, implode() only uses the values so this may result in a higher cache hit rate.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet