This PR overhauls extension registration in order to make it more
modular.
The `extension` crate now contains an `ExtensionHostProxy` that can be
used to register various proxies that the extension host can use to
interact with the rest of the system.
There are now a number of different proxy traits representing the
various pieces of functionality that can be provided by an extension.
The respective crates that provide this functionality can implement
their corresponding proxy trait in order to register a proxy that the
extension host will use to register the bits of functionality provided
by the extension.
Release Notes:
- N/A
This PR extracts the `ExtensionSlashCommand` implementation to the
`assistant_slash_command` crate.
The slash command related methods have been added to the `Extension`
trait. We also create separate data types for the slash command data
within the `extension` crate so that we can talk about them without
depending on the `extension_host` or `assistant_slash_command`.
Release Notes:
- N/A
