[CT-1891] Abstract graph generation from task classes

[stu-k]

The work in #6565 abstracted manifest generation from our task classes to a decorator to a click function. In the same way, we should abstract graph generation (i.e. ConfiguredTask.compile_manifest and BuildTask.compile_manifest) to a click decorator. This allows us to modify the functionality for the graph generation (currently occurring in BuildTask, hence its overriding of compile_manifest) task by task rather than having that modification present in the task itself.