Add CachingOHttpCryptoProvider to reduce overhead

[normanmaurer]

Motivation:

Creating HPKE*Context instances is expensive. If the same configuration is used we should be able to share the same instance

Modifications:

Add CachingOHttpCryptoProvider which caches and shares HPKE*Context instances when possible

Result:

Less overhead

[normanmaurer]

This needs tests

[thomdev]

Looks great! Nice trick with the back-and-forth conversions between closable and reference-counted objects. There are security implications with the caching of the sender context since it contains an ephemeral key. It could be worth mentioning in a comment.

[normanmaurer]

@thomdev thanks for the comments... do you think it would be better to only cache the recipient ?

[thomdev]

Actually I don't think that caching will work since the enc parameter, which is produced by a randomized algorithm on the sender, will be different every time for the recipient.

[normanmaurer]

@thomdev so if I understand correctly we can not cache anything, correct ?

[thomdev]

The server could cache the output of EVP_HPKE_KEY_new_and_init_or_throw.

[normanmaurer]

@thomdev #23 ;) ... Let me close this one and rebase + merge #23