[DC] Parse DC create response to create a DigitalCredential object

In the IdentityCredentialManager API for create() we return a string
with a json. This CL adds the logic to convert that string to a
DigitalCredential object so that we can return it to the Java client.

This is aligned with latest Web API spec and Android API spec.

Bug: 415334530
Change-Id: I630485325de03f36071075d3f2d880a67091d0bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6531080
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1458161}