Deterministic signatures

The OpenSSL-based deterministic signing implementation is likely very temporary, as we may switch to libsecp256k1 for signing soon (#5220), but it allows testing. The signatures generated by the OpenSSL and libsecp256k1 based implementations should be identical, so the test cases are already useful.

New format would result in ``BITCOIN_CRYPTO_HMAC_SHA256_H`` I guess you should look at the oder headers and end comments, too :). I just will give this single nit, nice, isn't it ^^?