Add require_secure_transport

[kolbe]

Feature Request

Is your feature request related to a problem? Please describe:

We should add support for a require_secure_transport startup option that matches the behavior of MySQL Server:

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_require_secure_transport

https://dev.mysql.com/doc/refman/5.7/en/using-encrypted-connections.html#mandatory-encrypted-connections

Describe the feature you'd like:

"With require_secure_transport enabled, client connections to the server are required to use some form of secure transport, and the server permits only TCP/IP connections that use SSL, or connections that use a socket file (on Unix) or shared memory (on Windows). The server rejects nonsecure connection attempts, which fail with an ER_SECURE_TRANSPORT_REQUIRED error."

Describe alternatives you've considered:

Without a require_secure_transport setting, an administrator must be careful to create all users with a REQUIRE TLS/ISSUE/CIPHER/etc. clause.