Add more system settings

[systay]

Building on the experimental SET support for session system settings added in #6459:
System variables have been put into four different buckets:

• Ignore These settings are accepted and ignored
• Error Attempting to SET these will always result in an error
• CheckAndIgnore For these settings, Vitess will accept the SET statement only when the value matches what is already configured globally.
• UseReserveConn This is the exciting new feature we’ve added - if these settings are changed, a dedicated connection will be used to the underlying database.