Adds ability to fork application via initialize_from

[skrawcz]

This gets at #181, by enabling someone
to fork an application easily from prior state.

You don't need to manage this outside of Burr,
it can now be done with the existing persisters.

I have included various error checks, just need
to write some tests for them.

Changes

• ApplicationBuilder to take in some more parameters
• updated conversational RAG notebook to show case the new feature

How I tested this

• locally

Notes

Checklist

• PR has an informative and human-readable title (this will be pulled into the release notes)
• Changes are limited to a single goal (no scope creep)
• Code passed the pre-commit check & code is left cleaner/nicer than when first encountered.
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future TODOs are captured in comments
• Project documentation has been updated if adding/changing functionality.

[elijahbenizzy]

Looks good, just some small comments on documentation