ENCODER: Refactor the OSSL_ENCODER API to be more like OSSL_DECODER

ENCODER: Refactor the OSSL_ENCODER API to be more like OSSL_DECODER

OSSL_ENCODER was developed before OSSL_DECODER, so the idea of
chaining and the resulting API came later.  This series of changes
brings the same sort of API and functionality back to OSSL_ENCODER,
making the two APIs more consistent with each other.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/12873)