Add ModelBroker opt-guide interface.

* Adds a ModelBroker mojom interface for on-device models from
  opt-guide.
  * Implemented by ODMServiceController.
  * Will later be used a Renderer->Browser interface.
* Adds ModelBrokerClient for constructing
  OptimizationGuideModelExecutor::Sessions based on that.

Design: https://docs.google.com/document/d/1rCq6oQdQLfY72s0uZj8SeW6_PGefk7-dNfWSZsXMhx4/edit?tab=t.0#heading=h.tyz8nxrnqsta

Bug: 403352722

Change-Id: If1f72310fedc0893057b49a18da49adb21d09f65
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6432711
Reviewed-by: Matthew Denton <mpdenton@chromium.org>
Reviewed-by: Fred Shih <ffred@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1446054}