Install or test plugin from public repository

[pauldix]

We are setting up a public plugin repository at https://github.com/influxdata/influxdb3_plugins. Users should be able to specify a plugin by name from that repo.

Let's overload the filename argument currently being used. If it starts with gh: then the remainder should be parsed as a location in that repo. So the plugin at https://github.com/influxdata/influxdb3_plugins/blob/main/examples/wal_plugin/wal_plugin.py would be loaded by specifying a filename of gh:examples/wal_plugin.

The server should go to the main branch of that repo and load in the file wal_plugin.py. Plugins in that repo will always have the same filename as the parent directory.