python: use plugins if available in the jobspec validator

Problem: Job submission cli plugins offer a validate() callback,
but this callback is not used.

Load plugins and run all validate callbacks in the jobspec validator
plugin.

src/bindings/python/flux/job/validator/plugins/jobspec.py

@@ -17,13 +17,15 @@
 
 import json
 
+from flux.cli.plugin import CLIPluginRegistry
 from flux.job import validate_jobspec
 from flux.job.validator import ValidatorPlugin
 
 
 class Validator(ValidatorPlugin):
     def __init__(self, parser):
         self.require_version = 1
+        self.plugins = []
         parser.add_argument(
             "--require-version",
             metavar="V",
@@ -47,5 +49,12 @@ def configure(self, args):
                 raise ValueError(f"Invalid argument to --require-version")
             self.require_version = None
 
+        self.plugins = CLIPluginRegistry().load_plugins("validate")
+
     def validate(self, args):
-        validate_jobspec(json.dumps(args.jobspec), self.require_version)
+        result, jobspec = validate_jobspec(
+            json.dumps(args.jobspec), self.require_version
+        )
+
+        # validate with any submit cli plugin validate methods
+        self.plugins.validate(jobspec)