serve workflow templates from custom_nodes

[bezo97]

This PR prepares the following feature request: Comfy-Org/ComfyUI_frontend#1008
Related frontend PR: Comfy-Org/ComfyUI_frontend#2032

Implementation details:

• @routes.get("/workflow_templates") - This endpoint returns a map of custom_nodes names and their associated workflow template files. The ones without templates are omitted. Example:

{
    "ComfyUI-LTXTricks": [
        "example_ltxv_stg",
        "example_ltx_flow_edit",
        "example_ltx_interpolation",
        "example_ltx_inversion",
        "example_ltx_iv2v",
        "example_ltx_rf_edit"
    ],
    "ComfyUI_Noise": [
        "unsample_example",
        "variations_example"
    ]
}

• LOADED_MODULE_DIRS where we keep track of all successfully loaded custom_nodes and their directories (even when there is no class mapping and web directory)
• files are served from each custom_nodes's specific subfolder if exists, like this: custom_nodes/*/example_workflows/*

Question to reviewers:

• Is serving from the /example_workflows folders sufficient or shall we support existing folder structures of popular custom_node repos by allowing /examples, /workflows, etc.?

[huchenlei]

We do have a plan to eventually move ComfyUI-Manager's custom node management features to core. Can we move this endpoint to a separate file app/custom_node_manager.py?

[huchenlei]

I think you can just call folder_paths.get_folder_paths("custom_nodes") here instead of adding a new global variable in folder_paths.