Add `"integrity"` to `GET /map/<name>/<version>`

[wkillerud]

Subresource integrity recently got added to importmap. Since Eik calculates the integrity hash of assets it would be nice to have it included alongside the "imports" key.

{
  "imports": {
    "square": "https://eik.store.com/npm/shapes/v1/square.js"
  },
  "integrity": {
    "https://eik.store.com/npm/shapes/v1/square.js": "sha384-oqVuAfXRKap7fdgcCY5uykM6+R9GqQ8K/uxy9rx7HNQlGYl1kPzQho1wx4JwY8wC"
  }
}

The workaround with today's API is to:

• iterate over all entries in "imports"
• do a GET for the value of the import up to and including the version number/alias (https://eik.store.com/npm/shapes/v1, follow redirects)
• iterate over that response's "files" to find the correct asset
• use "integrity" from that entry in "files" to build the "integrity" map