python: extend submission cli commands with plugins

Problem: Flux submission cli commands don't support plugins.

Load any plugins in the default plugin path
"{confdir}/cli/plugins/*.py" and call any plugin `add_options()` and
`modify_jobspec()` callbacks at appropriate points.

src/bindings/python/flux/cli/base.py

@@ -36,6 +36,7 @@
 
 import flux
 from flux import debugged, job, util
+from flux.cli.plugin import CLIPluginRegistry
 from flux.constraint.parser import ConstraintParser, ConstraintSyntaxError
 from flux.idset import IDset
 from flux.job import JobspecV1, JobWatcher
@@ -741,7 +742,9 @@ def __init__(self, prog, usage=None, description=None, exclude_io=False):
         self.exitcode = 0
         self.progress = None
         self.watcher = None
+        self.plugins = CLIPluginRegistry().load_plugins(prog)
         self.parser = self.create_parser(prog, usage, description, exclude_io)
+        self.plugins.add_options(self.parser)
 
     @staticmethod
     def create_parser(
@@ -1000,6 +1003,9 @@ def jobspec_create(self, args):
         """
         Create a jobspec from args and return it to caller
         """
+        # allow plugins to initialize args
+        self.plugins.preinit(args)
+
         jobspec = self.init_jobspec(args)
         jobspec.environment = get_filtered_environment(args.env)
         jobspec.cwd = args.cwd if args.cwd is not None else os.getcwd()
@@ -1111,6 +1117,8 @@ def jobspec_create(self, args):
             for arg in args.add_file:
                 self.handle_add_file_arg(jobspec, arg)
 
+        self.plugins.modify_jobspec(args, jobspec)
+
         return jobspec
 
     def submit_async(self, args, jobspec=None):