CXXCBC-567: Add support for field-level encryption library

[DemetrisChr]

Motivation

In order to support a separate Field Level Encryption library we need to provide a way to specify a pointer to a "crypto manager" which is then passed on to special transcoders which can handle encryption/decryption. The definition of the crypto transcoder class will be provided by the separate FLE library.

Changes

• Add forward declaration for couchbase::crypto::manager
• Add crypto manager in cluster options & pass that on to the cluster/bucket/scope/collection classes
• Add couchbase::codec::is_crypto_transcoder transcoder trait
• For transcoders that satisfy couchbase::codec::is_crypto_transcoder, call their encode/decode methods with an additional parameter, the crypto manager pointer.
• Add an internal (not intended for use by any users other than the FLE library) header file with helpers for aead_aes_256_cbc_hmac_sha512 encryption/decryption and generating secure random initialization vectors. These utilize the relevant BoringSSL/OpenSSL functions internally, and allow the FLE library to use them without adding a direct dependency to BoringSSL or OpenSSL