Implement loading MemoryItems from file in JSONFileMemory

[Pwuts]

Background

JSONFileMemory will write memories to a file, but can't yet load them back from a file. This PR fixes that.

Changes

• Implement loading MemoryItems from JSON index file
• Remove init param from get_memory(), replace usages by memory.clear()
• Make token length calculation optional in MemoryItem.dump()

Documentation

x

Test Plan

I added a test for JSONFileMemory.load_index(), so if CI is green we're good

PR Quality Checklist

• My pull request is atomic and focuses on a single change.
• I have thoroughly tested my changes with multiple different prompts.
• I have considered potential risks and mitigations for my changes.
• I have documented my changes clearly and comprehensively.
• I have not snuck in any "extra" small tweaks changes.
• I have run the following commands against my code to ensure it passes our linters:

  black .
  isort .
  mypy
  autoflake --remove-all-unused-imports --recursive --ignore-init-module-imports --ignore-pass-after-docstring autogpt tests --in-place

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