C-KZG-4844: A minimal library for EIP-4844 Polynomial Commitments

This is a copy of C-KZG stripped down to support the Polynomial Commitments API:

• compute_aggregate_kzg_proof
• verify_aggregate_kzg_proof
• blob_to_kzg_commitment
• verify_kzg_proof

We also provide load_trusted_setup and free_trusted_setup to load the trusted setup data from a file into an object that can be passed to the API functions, and conversions to/from byte arrays for the relevant types.

The only dependency is blst. Ensure blst.h is provided in inc and libblst.a in lib. (blst.h includes blst_aux.h, but the latter is unused and can be empty.) TODO: import these via git submodule