Make `CipherCtx::cipher_update` more flexible

This change relaxes constraints on the output buffer size when it can be
safely determined how many bytes will be put in the output buffer.

For supported cryptographic backends (OpenSSL >= 1.1) the cipher's `num`
parameter will be consulted for the number of bytes in the block cache.
For unsupported backends the behavior will not change (the code will
assume full block in the cache).

For callers that do the check themselves and want to use other backends
(e.g. BoringSSL or LibreSSL) and unsafe `cipher_update_unchecked`
function is added.

Additionally a `CipherCtx::minimal_output_size` function is added for
letting the callers know how big should the output buffer be for the
next `cipher_update` call.

Fixes #1729.

See: https://mta.openssl.org/pipermail/openssl-users/2022-November/015623.html