ENCODER: Add output structure support for EVP_PKEY encoding

ENCODER: Add output structure support for EVP_PKEY encoding

OSSL_ENCODER_CTX_new_by_EVP_PKEY() takes one more argument to express
the desired outermost structure for the output.

This also adds OSSL_ENCODER_CTX_prune_encoders(), which is used to
reduce the stack of encoders found according to criteria formed from
the combination of desired selection, output type and output
structure.

squash! ENCODER: Add output structure support for EVP_PKEY encoding

Replace the paragraph talking about OSSL_ENCODER_CTX_prune_encoders() with:

The encoding processor encoder_process() is enhanced with better
analysis of the stack of encoder implementations.  To avoid having to
keep an on the side array of information, it uses recursion.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/13167)