Refactor: Gthulhu plugin should follow singleton pattern

[ianchen0119]

The file https://github.com/Gthulhu/plugin/blob/main/plugin/gthulhu/sched.go
defines a lot of global variables used by GthulhuPlugin, defined in https://github.com/Gthulhu/plugin/blob/main/plugin/gthulhu/gthulhu.go

All of the APIs and global vars here should be embedded in the struct GthulhuPlugin.
Hence, the developer can create a GthulhuPlugin instance and load it to the Gthulhu scheduler.