Implement OSSL_Encoder and OSSL_Decoder APIs

[marcoh00]

Functionality for importing and exporting keys currently relies heavily on the PEM_{read,write}_bio_* and {i2d,d2i}_* type of functions (as an example, see RSA private key export and import). However, these functions were deprecated with OpenSSL 3.0 ([1] [2]).

As a replacement, the documentation references the OSSL_Encoder and OSSL_Decoder APIs. Apart from making this library a bit more future-proof, the new API looks quite flexible to me and it should be quite easy to realize functionality like exporting keys to various formats (including PKCS#8, which would cover the use case that was prototyped in #1374).

This probably involves a bit of work, so just see this as a tracking issue. I could imagine to work on this in case of positive feedback and an outlook that this could in fact be merged but unfortunately I cannot promise anything as I don't have a lot of time on my hands and I have no idea how much work this actually is.