feat: Make fetchBlob() use Cross-Origin Storage when available, else fall back to Cache API for Wasm / JS caching

[tomayac]

On the Chrome team, we're considering a new browser API and storage mechanism called Cross-Origin Storage (COS), which would allow for resources (like large Wasm files as in the ffmpeg.wasm project's case) that are identified by their hashes to be shared across origins. (Dear maintainers: @lucasgelfond, @jeromewu, feel free to reach out directly if you have questions, tomac AT google . com.)

The present PR adds support for this proposed API as a progressive enhancement, falling back to the Cache API. It also adds a simple "hello world" app that shows the progressive enhancement and the caching in both cases in action. You can test the COS API by installing the COS extension. Even without the extension installed, the PR improves upon the status quo by adding default caching.

Regular case:

Caching the resources in the Cache API, isolated per origin:

Cross-Origin Storage case:

Caching the resources in Cross-Origin Storage, shared across origins (see the popup window of the extension, the ffmpeg-core.wasm file is shared between http://localhost:3000 and https://tomayac.github.io):

With the extension installed, after opening the first demo link, try the same demo on another origin and you should see how the ffmpeg-core.wasm resource (and the JavaScript) is shared.

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