Implement Glic panel coordination logic

This change provides the initial implementation for the
`GlicPanelCoordinator`. It introduces the core logic for managing
the lifecycle of `GlicInstance` objects.

Key features include:
- A `GlicInstance` now manages its own UI embedder, determined by an
  `EmbedderType` (side-panel or floating).
- The coordinator can manage multiple attached instances and a single
  floating instance.
- Implements the primary user actions: toggling a panel for a
  specific browser, attaching a floating panel, and detaching a
  panel to make it float.

Bug: b:441793618
Change-Id: I6a6a6964340ef624931c0515f4b325e885eceaf5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6896815
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: Ian Wells <iwells@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1509842}