This PR adds a new `extension` crate, containing some contents extracted
from the `extension_host`.
Right now it contains just the `ExtensionManifest` and
`ExtensionBuilder`, although we may move more of the extension interface
into here.
The introduction of the `extension` crate allows us to depend on it in
the `extension_cli`, thereby eliminating the need for the `no-webrtc`
feature on a number of crates.
Release Notes:
- N/A
This PR renames the `extension` crate to `extension_host`.
This is to free up the name so that we can create a smaller-scoped
`extension` crate.
Release Notes:
- N/A
