[DC] Add use counters for Digital Credential protocols.

This changelist adds use counters to track the usage of different
protocols for the Digital Credentials API.

The key changes are:
- New `WebFeature` enums have been added to `web_feature.mojom` and
`tools/metrics/histograms/metadata/blink/enums.xml` for different
digital credential protocols (e.g., `OpenId4VpUnsigned`, `OpenId4Vci`).

- In `digital_identity_credential.cc`, a new function
`RecordProtocolUseCounters` is introduced to increment the appropriate
use counter based on the protocol string from the API call.

- This function is called for both `get` (`Discover...`) and `create`
(`Create...`) credential flows.

- A new test file, `digital_identity_credential_protocol_test.cc`,
has been added to verify that the use counters are correctly recorded
for various protocols.

Fixed: 447566717
Change-Id: If92ac88be9f2cd288fc31c07a10b1f2dc24396b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7252371
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1558278}