Migrate prompt cache schema to YAML

[sarda-nikhil]

There's plenty of evidence showing that yaml is a pretty good candidate for hitting the sweet spot of being both readable and succinct.

[shsym]

I have been using YAML, XML, and JSON for various configuration files. Thinking that XML is too descriptive and complicated, I usually prefer JSON over YAML since the indent-based formatting in YAML created a lot of headaches in my use cases, especially for a collaborative environment where people/students do not have a concrete agreement on indentation (for example; maybe company environment can be different, but in academic research, I think people are more liberal ;) )