[feature] Add support for kv assets.

[armfazh]

It exposes env.asset_key function so filenames can be queried from the "__STATIC_CONTENT" KV namespace.

Previously, names were mangled due to an additional hash appended to the names.
Now, inside the worker handler one can access to favicon.ico as (simplified):

let key = ctx.env.asset_key("favicon.ico");
let icon = ctx.kv("__STATIC_CONTENT")?.get(&key);

This PR depends on:

• workers-sdk: Include an empty __STATIC_CONTENT_MANIFEST if not defined. workers-sdk#3084
  A small change to always load a (possibly empty) manifest module.
• miniflare: Add module __STATIC_CONTENT_MANIFEST miniflare#622
  For running tests.
• workerd: Detects __STATIC_CONTENT_MANIFEST as a special module. workerd#828
  For running tests.

Once these PRs get merged, CI will pass.

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Credits: This code was based on @SeokminHong comment in #54

[zebp]

I'm unsure if this deserves to be merged into the workers-rs framework itself. Like the existing TypeScript kv-asset-handler this would be better served as a standalone library IMO.

That being said, I'm not totally against adding first party support if there's a good reason as to why it should be in the official crate.

[penalosa]

@zebp From the wrangler/miniflare side I tend to agree. Use of Workers Sites is discouraged in favour of Pages, and the assets story in Wrangler will be revisited as part of Pages/Workers convergence in the near future. As such, adding additional functionality to Workers Sites is generally not something we want to do, and introducing a new API directly to the workers-rs framework for this seems like a maintenance footgun in the not too distant future.