SipHash: add separate setter for the hash size

This was originally part of SipHash_Init.  However, there are cases
where there isn't any key material to initialize from when setting the
hash size, and we do allow doing so with a EVP_PKEY control.  The
solution is to provide a separate hash_size setter and to use it in
the corresponding EVP_PKEY_METHOD.

Fixes #7143

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7145)