[KVStore] Improve the `badger` submodule

[h5law]

Objective

Improve the badger submodule to make it more attractive to potential users

Goals

• Improve the documentation
• Improve the BadgerKVStore interface and wrapper in general
• Research other KVStore implementations for desired features

Deliverables

• Add runnable examples in the codebase
• Add more features to the badger submodule's KVStore
• Expose more of Badger v4's own features

Non-goals / Non-deliverables

• Break compatibility with the MapStore interface
• Disable its usage as a nodestore for the SMT

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit/fuzz/benchmarks) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @h5law