Add a generic SubjectPublicKeyInfo decoder

Add a generic SubjectPublicKeyInfo decoder

Previously all the SubjectPublicKeyInfo decoders were specific to a key
type. We would iterate over all them until a match was found for the correct
key type. Each one would fully decode the key before then testing whether
it was a match or not - throwing it away if not. This was very inefficient.

Instead we introduce a generic SubjectPublicKeyInfo decoder which figures
out what type of key is contained within it, before subsequently passing on
the data to a key type specific SubjectPublicKeyInfo decoder.

Fixes #15646

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15662)