Add new extension crate (#20089)

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

crates/workspace/Cargo.toml

@@ -13,7 +13,6 @@ path = "src/workspace.rs"
 doctest = false
 
 [features]
-no-webrtc = ["call/no-webrtc"]
 test-support = [
     "call/test-support",
     "client/test-support",