feat: serialization to and from felts

[xJonathanLEI]

Adds initial support for serialization of Rust types into Felts as well as deserialization.

This PR adds two new traits as starknet_core::codec::Encode and starknet_core::codec::Decode. Common Rust primitive types such as u64 and Option<T> implement these traits, following the same rules as the Serde trait in Cairo.

These two traits are derivable. Users can simple write something like:

#[derive(Encode, Decode)]
struct CairoType {
    a: Felt,
    b: Option<u32>,
    c: bool,
}

A new example serde has been added to demonstrate the usage of these new building blocks.

This is useful for constructing contract calldata and interpreting contract call responses fetched from JSON-RPC. Previously, users would have to manually decode the raw Felts into proper Rust types.

For now, users still need to manually write the ABI types in Rust. The next step is to add support for generating such types from a contract ABI.