Add SSZ Poseidon2 wrapper and document SSZ usage

[chetanyb]

Description:

• Adds an SSZ‑compatible Poseidon2 wrapper under src/ssz/ with a SHA256‑style API.
• Exports hash_zig.ssz.SszHasher (Poseidon2‑24) and runs Plonky3 parity tests.
• Documents SSZ integration in the README with a minimal usage example.

NOTE: version bump post‑merge

[ch4r10t33r]

Have you validated if the cross language tests are working correctly post this change? Please test this for all lifetimes (2^8, 2^18 and 2^32) with number of active epochs as 256/1024.

[ch4r10t33r]

Looks good.