feat(core): allow users to access the Assets instance

[lucasfernog]

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• New Binding Issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes. Issue #___
• No

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix: #xxx[,#xxx], where "xxx" is the issue number)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

Allows Tauri users to access the assets instance, e.g.

fn main() {
  use tauri::Assets;
  let context = tauri::generate_context!();
  let assets = context.assets.clone();
  // now you can read the embedded assets anywhere in the code
  let asset = assets.get("something");
  tauri::Builder::default()
    .run(context)
    .expect("error while running tauri application");
}

[chippers]

This change looks fine to me, I don't see any harm in constructing the Arc externally for easy Arc<EmbeddedAssets>. It does currently implement Send + Sync + 'static so references to it should probably work to pass to other things like web servers although I haven't personally tested it.

edit: this is a breaking change so that should be mentioned in the changefile

[lucasfernog]

Using the reference doesn't work since it expects the context to be static. So this change is needed.