Improve Dolt sysvar support

There are many MySQL of system variables like `@@max_user_connections`, `@@max_connections`, `@@port`. A full list can be found here:

https://dev.mysql.com/doc/refman/8.0/en/dynindex-sysvar.html

Dolt does not set many of these and for some returns non-sensical values. For instance we always return 3306 for `@@port` even if you've set a different port (#5898).

We need to run a project tio go through that very long list, set the ones we can, and finally document what we support and do not support under:

https://docs.dolthub.com/sql-reference/sql-support