[hd, hdsi, hdGp] Scene index plugin ordering via JSON metadata

This change introduces a JSON metadata-based system for declaring scene index plugin ordering, as an alternative and improvement to the existing C++ registration API.
It lays the groundwork for eventually deprecating the C++ registration API in favor of a more declarative, data-driven approach.

Plugins can now declare ordering constraints directly in their plugInfo.json via the following new fields:
  - tags - a list of semantic labels for a plugin (the plugin's own ID is added as a default tag)
  - ordering.before / ordering.after - tag-based constraints specifying which groups of plugins this plugin must come before or after
  - ordering.position - a tiebreaker within the unconstrained middle group (firstAfter, lastBefore, doesNotMatter)

A new PluginOrderingPolicy enum controls which ordering mechanism is active:
  - CppRegistrationOnly (default) - preserves existing behavior, uses C++ insertion phase/order
  - JsonMetadataOnly - uses JSON metadata exclusively
  - Hybrid - attempts to honor both C++ registration order and JSON metadata constraints

When using JsonMetadataOnly or Hybrid, ordering constraints are resolved using a topological sort (Kahn's algorithm).

The default policy can be set via the env var HD_SCENE_INDEX_PLUGIN_ORDERING_POLICY_DEFAULT.
It can also be set programmatically via SetPluginOrderingPolicy(), but this API is intended for testing purposes.

Additionally, this change addresses a latent bug wherein the app name was used as a filter for plugin loading, but not when gathering the plugin entries.

Callouts:
Previously, plugins could use an env var to guard C++ registration causing the plugin to be excluded when creating the scene index plugin graph. Plugins should now also use the guard in _AppendSceneIndex so that the input scene is returned if the plugin shouldn't have an effect. See changes in hdsi/hdGp for examples.

Testing:
Updated testHdSceneIndexPluginRegistry to exercise all 3 ordering policies.

(Internal change: 2398753)

Author: rajabala

pxr/imaging/hd/debugCodes.cpp

@@ -83,6 +83,9 @@ TF_REGISTRY_FUNCTION(TfDebug)
     TF_DEBUG_ENVIRONMENT_SYMBOL(HD_SAFE_MODE,
         "Enable additional security checks");
 
+    TF_DEBUG_ENVIRONMENT_SYMBOL(HD_SCENE_INDEX_PLUGIN_REGISTRY,
+        "Report debug info on scene index plugin registry");
+
     TF_DEBUG_ENVIRONMENT_SYMBOL(HD_SELECTION_UPDATE,
         "Report when selection is updated");
 

pxr/imaging/hd/debugCodes.h

@@ -43,6 +43,7 @@ TF_DEBUG_CODES(
     HD_RPRIM_REMOVED,
     HD_RPRIM_UPDATED,
     HD_SAFE_MODE,
+    HD_SCENE_INDEX_PLUGIN_REGISTRY,
     HD_SELECTION_UPDATE,
     HD_SHARED_EXT_COMPUTATION_DATA,
     HD_SPRIM_ADDED,