Implements Hybrid Public Key Encryption (HPKE) as per RFC9180.

This supports all the modes, suites and export mechanisms defined
in RFC9180 and should be relatively easily extensible if/

Implements Hybrid Public Key Encryption (HPKE) as per RFC9180.

This supports all the modes, suites and export mechanisms defined
in RFC9180 and should be relatively easily extensible if/as new
suites are added. The APIs are based on the pseudo-code from the
RFC, e.g. OSS_HPKE_encap() roughly maps to SetupBaseS(). External
APIs are defined in include/openssl/hpke.h and documented in
doc/man3/OSSL_HPKE_CTX_new.pod. Tests (test/hpke_test.c) include
verifying a number of the test vectors from the RFC as well as
round-tripping for all the modes and suites. We have demonstrated
interoperability with other HPKE implementations via a fork [1]
that implements TLS Encrypted ClientHello (ECH) which uses HPKE.

@slontis provided huge help in getting this done and this makes
extensive use of the KEM handling code from his PR#19068.

[1] https://github.com/sftcd/openssl/tree/ECH-draft-13c

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17172)

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