Rename extension crate to extension_host (#20081)

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

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 = { workspace = true, features = ["no-webrtc"] }
+extension_host = { workspace = true, features = ["no-webrtc"] }
 fs.workspace = true
 language.workspace = true
 log.workspace = true