chore: sqlite framework

[SantiagoPittella]

closes #2248 and #2249

Summary

Introduces a light SQLite framework in crates/db (miden-node-db) and migrates the validator onto it, removing Diesel. It handles a connection pool, type-enforced read/write transactions, always-cached queries, a type codec, and cacheable IN-list helpers.

• The framework keeps a small pool of connections alive and hands them out. Because SQLite is blocking, the actual query work runs on a background thread.
• We dont handle raw connections. The struct provides either a read or a write transaction and get a small
  handle:
  • read(...) gives you a ReadTx. It begins a read-only transaction that is never committed, so
    nothing it does can persist.
  • write(...) gives you a WriteTx. It begins a write transaction.
• Added verbs like query_rows, query_opt, query_one, exists, count and execute (write only). Every one of them prepares its statement through SQLite's cache.
• Added a codec to convert between native data types and sql's.

The usage is like this:

// a read
db.read("chain_tip", |tx| {
    tx.query_opt(
        "SELECT block_header FROM block_headers ORDER BY block_num DESC LIMIT 1",
        &[],
        |row| row.get::<BlockHeader>(0),
    )
}).await?;

// a write (several statements in one atomic transaction)
db.write("apply", move |tx| {
    tx.execute("INSERT INTO ... VALUES (?1, ?2)", &[&a, &b])?;
    tx.execute("UPDATE ... SET ... WHERE id = ?1", &[&id])?;
    Ok::<(), DatabaseError>(())
}).await?;

Changelog

changelog = "none"
reason    = "Internal change only."