Selected the [Botan crypto library](https://github.com/randombit/botan) due to its feature list, maintainer support, availability across all deployment platforms, and ease of use. Also evaluated Crypto++ as a viable candidate, but the additional features of Botan (PKCS#11, TPM, etc) won out.
The random number generator received a backend upgrade. Botan prefers hardware-based RNG's and will provide one if available. This is transparent to KeePassXC and a significant improvement over gcrypt.
Replaced Argon2 library with built-in Botan implementation that supports i, d, and id. This requires Botan 2.11.0 or higher. Also simplified the parameter test across KDF's.
Aligned SymmetricCipher parameters with available modes. All encrypt and decrypt operations are done in-place instead of returning new objects. This allows use of secure vectors in the future with no additional overhead.
Took this opportunity to decouple KeeShare from SSH Agent. Removed leftover code from OpenSSHKey and consolidated the SSH Agent code into the same directory. Removed bcrypt and blowfish inserts since they are provided by Botan.
Additionally simplified KeeShare settings interface by removing raw certificate byte data from the user interface. KeeShare will be further refactored in a future PR.
NOTE: This PR breaks backwards compatibility with KeeShare certificates due to different RSA key storage with Botan. As a result, new "own" certificates will need to be generated and trust re-established.
Removed YKChallengeResponseKeyCLI in favor of just using the original implementation with signal/slots.
Removed TestRandom stub since it was just faking random numbers and not actually using the backend. TestRandomGenerator now uses the actual RNG.
Greatly simplified Secret Service plugin's use of crypto functions with Botan.
* Significantly improve user experience when using hardware keys on databases in both GUI and CLI modes. Prevent locking up the YubiKey USB interface for prolonged periods of time. Allows for other apps to use the key concurrently with KeePassXC.
* Improve messages displayed to user when finding keys and when user interaction is required. Output specific error messages when handling hardware keys during database read/write.
* Only poll for keys when previously used or upon user request. Prevent continuously polling keys when accessing the UI such as switching tabs and minimize/maximize.
* Add support for using multiple hardware keys simultaneously. Keys are identified by their serial number which prevents using the wrong key during open and save operations.
* Fixes#4400
* Fixes#4065
* Fixes#1050
* Fixes#1215
* Fixes#3087
* Fixes#1088
* Fixes#1869
