Add verifiable credentials flow in frontend

[nmattia]

This adds a "verifiable credentials" flow to the frontend. In four commits:

1. The canister code is updated to accept 2 index.js files; without this, the browser will refuse to fetch the new index file for the second flow due to bad CSP.
2. Adds the actual VC postMessage interface, flow & screens to the II webapp.
3. Updates the test-app by introducing a few react component to send & receive VC related requests to an IDP (II).
4. A couple of tests are added to go through a full VC flow.

This is more of a "clean proof-of-concept" than an actual feature, as many things are still missing:

• The VC flow does not support PIN identities
• The SRI hash fix is a bit ugly and should probably be fixed more generally (for 2+ index files)
• The issuer types are duplicated between the issuer app and II; moreover in both cases they include both the actual Issuer API (specced) and the helper API like add_employee. This should be cleaned up somehow.
• The tests merely checks that the flow is successful; they should also check the validity of the presentation and be extended to test for non-happy cases.
• There is no support for changing the consent message language
• The test-app requests are not typechecked (which might cause frustration if the types ever change)
• The consent message is not shown as rendered markdown

All of these will come in other PRs.

---

🟢 Some screens were added

🟡 Some screens were changed

[peterpeterparker]

Only had a look to the configuration and Rust code. Looks good, just few questions.

Rubber stamp for the JS/HTML, did not had a look.

[frederikrothenberger]

Thanks, LGTM!