Add `serde` to Merkle Tree proofs to enable Serialization/Deserialization

[bvrooman]

• [ ] Introduce serde feature flag to enable serde-specific compilation
• [ ] Include serde dependency in toml when compiling with serde feature flag
• [ ] Derive serde for SMT proofs when serde feature flag is enabled
• [ ] Derive serde for BMT proofs when serde feature flag is enabled (optional)

[MitchTurner]

This is blocked on us deciding what our main tree will be. Once we have deciding on the best, based on performance and our unique requirements, we can work on this.