Add built-in key-value store tools to persist data locally between `gptscript` invocations

[njhale]

Description

Add built-in tools that can store and retrieve arbitrary key-value pairs to/from disk on my local machine.

Motivation

If I want a tool to be able to reference info from a previous run of gptscript, the tool needs to persist that data somewhere. Today, I believe the onus of how and where to store and retrieve that data is placed on the tool author; e.g. I write another custom tool that writes to a specific file in some arbitrary format. Having a first class way to store key-value pairs would (probably) help me avoid rewriting such a tool every time.

[cjellick]

I'm going to classify this as medium primarily bc both Bill and Darren 👍 ed it. That tells me there's some validity (or was at some point), but I'm not sure it's still valid based on how gptscript has evolved. make sure @thedadams and @ibuildthecloud agree on this before implementing.