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/extension_host/src/extension_store_test.rs

@@ -1,4 +1,3 @@
-use crate::extension_manifest::SchemaVersion;
 use crate::extension_settings::ExtensionSettings;
 use crate::{
     Event, ExtensionIndex, ExtensionIndexEntry, ExtensionIndexLanguageEntry,
@@ -8,6 +7,7 @@ use crate::{
 use assistant_slash_command::SlashCommandRegistry;
 use async_compression::futures::bufread::GzipEncoder;
 use collections::BTreeMap;
+use extension::SchemaVersion;
 use fs::{FakeFs, Fs, RealFs};
 use futures::{io::BufReader, AsyncReadExt, StreamExt};
 use gpui::{Context, SemanticVersion, TestAppContext};