[browser] Allow downloading WebAssembly resources without performing other WebAssembly runtime initalization

[pavelsavara]

Fixes #100299

import { dotnet } from './_framework/dotnet.js'

// optional call to download all assets
await dotnet.download();

// and later it could be followed by usual
const runtime = await dotnet.create();

Note that when we download assets during cold start, they are being loaded into WASM memory as they arrive.
When you already have pre-downloaded the assets with this new API, the startup of the runtime will be more of the burst. Similar as with assets already in a cache.

[dotnet-policy-service]

Tagging subscribers to 'arch-wasm': @lewing
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[maraf]

Please add a WBT. Since we will remove custom cache shortly, we can check that all URLs that are used during "download" phase will be used during real start phase.

[pavelsavara]

Please add a WBT. Since we will remove custom cache shortly, we can check that all URLs that are used during "download" phase will be used during real start phase.

I'm not sure that I follow. What would the test look like ?

[maraf]

I'm not sure that I follow. What would the test look like ?

What I'm thinking about for the test is that we can ensure that no new resource will be requested during real start after download() was called. So the idea is to install resourceLoader (or override fetch), record all URLs used during download() and verify that no new URLs were requested during create()