SERIALIZER: add functions for serialization to file

SERIALIZER: add functions for serialization to file

These functions are added:

- OSSL_SERIALIZER_to_bio()
- OSSL_SERIALIZER_to_fp() (unless 'no-stdio')

OSSL_SERIALIZER_to_bio() and OSSL_SERIALIZER_to_fp() work as wrapper
functions, and call an internal "do_output" function with the given
serializer context and a BIO to output the serialized result to.

The internal "do_output" function must have intimate knowledge of the
object being output.  This will defined independently with context
creators for specific OpenSSL types.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10394)