DOLT_AUTHOR_DATE requires an equivalent session variable

Currently `dolt commit` and `dolt cherry-pick` (possibly other commands too) can set the date of the commit using the DOLT_AUTHOR_DATE, which works fine when using those CLI commands in local mode.

When running with a remote server though, or when attempting to perform the equivalent behavior when connected to a `sql-server` those environment variables have no effect.

Add a system variable which has the same effect, and update the CLI commands to set that variable when appropriate.

Trace how DOLT_AUTHOR_DATE is handled by the `dolt commit` and `dolt cherry-pick` entry points, then follow the corresponding session-variable path used by the SQL server and remote mode. Done means an equivalent system variable controls commit dates through SQL-server connections and the CLI commands set it when appropriate.

Written by the indexing model from the issue text.