Bug 1633866 - [taskgraph] Create a system to choose how test manifests are loaded via a parameter, r=tomprince

Currently test manifests are loaded by instantiating a TestResolver and
traversing moz.build files to find manifests.

With 'manifest-scheduling', we'll want to grab the manifests directly from the
bugbug service instead. Initially we'll want to enable manifest-scheduling with
|mach try auto|, but not on autoland yet.

This patch will allow |mach try auto| to set the parameter that causes bugbug
to be used (see future commits in this bug).

Differential Revision: https://phabricator.services.mozilla.com/D76522

taskcluster/taskgraph/parameters.py

@@ -98,6 +98,7 @@ base_schema = Schema({
     Required('signoff_urls'): dict,
     Required('target_tasks_method'): text_type,
     Required('tasks_for'): text_type,
+    Required('test_manifest_loader'): text_type,
     Required('try_mode'): Any(None, text_type),
     Required('try_options'): Any(None, dict),
     Required('try_task_config'): dict,
@@ -177,6 +178,7 @@ class Parameters(ReadOnlyDict):
             'signoff_urls': {},
             'target_tasks_method': 'default',
             'tasks_for': 'hg-push',
+            'test_manifest_loader': 'default',
             'try_mode': None,
             'try_options': None,
             'try_task_config': {},