Use ACP for Inline Assitant

[mpiannucci]

Currently you need to use the zed ai provider (or keys) for inline assistant. I would love to use an ACP provider for this instead

[yajo]

Docs say about it:

zed/docs/src/ai/inline-assistant.md

Lines 19 to 20 in 9da0d40

	> Unlike the Agent Panel, though, the only exception at the moment is [external agents](./external-agents.md).
	> They currently can't be used for generating changes with the Inline Assistant.

It says "at the moment", so hopefully some day this will be added 😊