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_cli/Cargo.toml

@@ -16,7 +16,7 @@ path = "src/main.rs"
 anyhow.workspace = true
 clap = { workspace = true, features = ["derive"] }
 env_logger.workspace = true
-extension_host = { workspace = true, features = ["no-webrtc"] }
+extension.workspace = true
 fs.workspace = true
 language.workspace = true
 log.workspace = true

crates/extension_cli/src/main.rs

@@ -9,7 +9,7 @@ use std::{
 use ::fs::{copy_recursive, CopyOptions, Fs, RealFs};
 use anyhow::{anyhow, bail, Context, Result};
 use clap::Parser;
-use extension_host::{
+use extension::{
     extension_builder::{CompileExtensionOptions, ExtensionBuilder},
     ExtensionManifest,
 };