Rework plugins so that it's only triggers

[pauldix]

Now that plugins come from either the local disk or Github, it's no longer necessary to have a plugin creation step separate from a trigger creation step. We can just create the trigger. I propose:

influxdb3 create trigger --plugin <path/foo/file.py> --type <wal|schedule|request|persist> --spec <the trigger spec> --arguments <comma separated k/v list>

The plugin part will either be a path to a file in the plugin dir, or start with gh: in which case it's to be found in the plugin repo in github.

All of the plugin commands should be removed.

It should still be possible to enable and disable triggers.