Support passing `user_agent` for layer repos / `kernelize`

[moon-bot-app]

Context

Downstream libraries (e.g. transformers, via integrations/hub_kernels.py) would like to identify which calls to the Hub originate from them, so we can understand what is driving kernel usage.

get_kernel() / install_kernel() already accept a user_agent (str | dict) that is forwarded to snapshot_download() (see kernels/src/kernels/load.py, install.py, hf_hub.py). However, the layer-repo path (LayerRepository + kernelize(), kernels/src/kernels/layer/) has no way to pass one — and that's the path transformers mostly uses (layer repo definitions), so downloads triggered through kernelize are currently unattributable.

Ask

• Allow wiring a user_agent through LayerRepository (and/or kernelize()) down to the Hub download calls, consistent with get_kernel().
• Keep it strictly optional / opt-in — telemetry has caused pushback before. HF_HUB_DISABLE_TELEMETRY already covers the huggingface_hub side, so respecting that is enough on our end.

Timing

Preferably land this after the kernel-to-kernel dependency work (kernels-as-dependencies, e.g. einops, CuTeDSL) merges, since how these arguments are passed around will change with it.

cc @danieldk

From a Slack discussion between Sayak Paul, Anton and Daniël de Kok.

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Requested by Sayak Paul - Slack thread - Agent trace