[metrics_internals] Show content for Private Metrics tab

This commit adds support for displaying content for the Private Metrics
tab at chrome://metrics-internals. The MetricsInternalsHandler is
registered as an observer to DwaService to get updates for
EncryptionPublicKey. The key is then converted to a dictionary and
passed to the frontend using the WebUIListener, using which, the
frontend renders the information in an HTML table.

Screenshots:
Before: https://screenshot.googleplex.com/4KxdZGbxQLF6Mfp.png
After: https://screenshot.googleplex.com/9RZTYRT2bw2Dxd2.png

Bug: b:445106658
Change-Id: I5f6378936810ed8e3ad06deeee5818b28ef12815
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7106466
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Elly FJ <ellyjones@chromium.org>
Commit-Queue: Chirag Arora <heychirag@google.com>
Cr-Commit-Position: refs/heads/main@{#1543237}
NOKEYCHECK=True
GitOrigin-RevId: 97f3472d99a92b6421db2330b76d781f98a30f99