Optimise PKEY decoders

Optimise PKEY decoders

The most expensive part of using a PKEY decoder is the
OSSL_DECODER_CTX_new_for_pkey() call. This builds up all of the decoder
chains, which is a complex and time consuming operation. However, if no
new providers have been loaded/unloaded since the last time it was called
we can expect the same results for the same parameters. Note that this
operation takes place *before* we event parse the data for decoding so it
is not dependent on the parsed data at all.

We introduce a cache for OSSL_DECODER_CTX objects. If we have been called
with the same parameters then we just duplicate an existing
OSSL_DECODER_CTX. This should be significantly faster than creating a new
one every time.

Partially addressed the issue in #15199

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21426)