Make option to background WAL plugin execution

[pauldix]

For WAL plugins, we should have an option that specifies if plugin runs should be backgrounded so that the next WAL flush can be handled regardless of how long the last one(s) take.

Some users may want these plugins to only ever execute strictly sequentially, so we should make this an option on the trigger definition.

Related Discord discussion: https://discord.com/channels/1325200757427015701/1325209754959872190/1334912417418707073