Storyteller.ai API integration (#49)

This PR integrates asset retrieval from the Storyteller.ai back-end API,
and begins implementing long-term scene saving and storage.

### Media retrieval
Instead of fetching raw asset URLs, going forward, media files from
remote sources (e.g., in URL parameters) should be specified with a path
like the following:
```
remote://<token>.<filetype>
```
where
* `token` is a media token (e.g., "m_70b77neq9jsgmy5v31mekzchcbd223")
* `filetype` is a file extension appropriate for the type of file being
  fetched (e.g., "gltf")

The `remote` protocol will be recognized by a [custom asset
reader](https://github.com/storytold/storyteller-bevy/blob/70726db111f6ccafd47feb055c5546540fef1786/studio/src/remote/reader.rs)
which will retrieve the file by calling a Storyteller API endpoint.

### Scene storage
Additionally, this PR begins laying the groundwork for
storing/retrieving Bevy scenes to/from the backend instead of solely
in-memory. This work is still incomplete -- as of this PR, a button has
been added to the web UI allowing the user to download the scene
file locally for testing/debugging purposes.

### Test URL
Media retrieval from the backend can be tested by [visiting this
URL](https://deploy-preview-49--storyteller-studio.netlify.app/?mode=viewer&mixamo=remote://m_70b77neq9jsgmy5v31mekzchcbd223.gltf)
in Chrome with the `--disable-web-security` flag (to bypass CORS errors)

studio/Cargo.toml

@@ -27,9 +27,9 @@ console_error_panic_hook = { version = "0.1.7", optional = true }
 itertools = "0.12.1"
 js-sys = { version = "0.3.65", optional = true }
 parking_lot = "0.12.1"
-serde = { version = "1.0.192", optional = true }
+serde = "1.0.192"
 serde-wasm-bindgen = { version = "0.6.1", optional = true }
-serde_json = { version = "1.0.108", optional = true }
+serde_json = "1.0.108"
 studio-bvh = { path = "../studio-bvh" }
 wasm-bindgen = "0.2.84"
 web-sys = { version = "0.3.65", optional = true, features = [
@@ -49,12 +49,18 @@ bytemuck = "1.14.1"
 space_editor = {git = "https://github.com/rewin123/space_editor", rev = "ce83f86dc902787ab16213f3209276a784654df6"}
 anyhow.workspace = true
 
+[dependencies.reqwest]
+version = "0.11.24"
+features = [
+	"default",
+	"json",
+	"stream",
+]
+
 [features]
 wasm = [
 	"dep:console_error_panic_hook",
 	"dep:js-sys",
-	"dep:serde",
-	"dep:serde_json",
 	"dep:serde-wasm-bindgen",
 	"dep:web-sys",
 	"bevy_rapier3d/wasm-bindgen",