Major fixes to authentication support

Fixes the following failure modes:

In tests, certain errors would crash instead of failing
The xor(_:_:) utility did not correctly apply the operation to the entire buffer.
Most protocol errors now throw a much more specific error.
Trace logging is now more verbose during authentication.
The AuthSwitchRequest packet (tagged with 0xfe during the authentication phase, incorrectly treated as EOF before) is now correctly handled; this permits both the use of arbitrary auth plugins and the use of the "fast auth" mechanism.
The caching_sha2_password handling would previously incorrectly treat the trailing NUL as part of the auth data.
The fast_auth_success (0x1 0x3) packet during authentication is now correctly handled.
When the caching_sha2_password plugin fails to authenticate on non-TLS connections, we now log a useful error message and throw a meaningful error. The framework is in place to implement this fully as soon as a usable implementation of the RSA public-key encrypt operation (with PEM armor and PKCS#1-OAEP padding support) is available.

This massively increases compatibility with all supported versions of MySQL and MariaDB and paves the way for further improvements.

Tagged semver-minor due to the new enumerators for the more specific errors.

vapor / mysql-nio