feat: first itereation of MVP

[damiramirez]

This PR introduces a first implementation of EthrexDB

• file_manager.rs: Handles file I/O and memory mapping
• serialization.rs: Converts between MPT nodes and binary format using a 2-node system (Branch + Extend)
• db.rs: Main API providing commit(), get(), and root() operations

In this first iteration, the DB depends on a root node, so I'm using ethrex::InMemoryTrieDB to handle the inserts. Once we finish adding values to the tree, we can call db.commit(&root_node) to serialize the entire MPT to a file.

Since we're using structs/function from Ethrex, the trie and RLP logic has been copied from the Ethrex repository. You can find this code under src/trie and src/rlp.

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Benchmarking and Profiling

To run a benchmark comparing this new implementation against libmdbx (as we do in Ethrex), run:

make bench

To profile using Samply, run:

make profile

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File Format

Implements a simple append-only file for storing complete MPT

[header: 8 bytes] -> points to latest version
[version 1: [prev_offset: 8 bytes][serialized_nodes]]
[version 2: [prev_offset: 8 bytes][serialized_nodes]]
...
[version N: [prev_offset: 8 bytes][serialized_nodes]] <- latest

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Limitations of this iteration - Next steps

• Does not support NodeRef::Hash references (supports only NodeRef::Node).
• Stores complete tries instead of incremental deltas.
• Add a new benchmark using Libmdbx with path
• Error handling.