WebAuthn: Include immediateGet in client capabilities

This CL adds the "immediateGet" client capability if the
`WebAuthenticationImmediateGet` flag is enabled. This will allow
websites to detect the support for immediate mediation.

We still need to figure out how this should behave during origin
trials, as enabling the feature flag will report true for all
origins.

Bug: 393055190
Change-Id: Idd8d9bab7ae2e98164f190181335e7a6429904fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6478730
Commit-Queue: Adem Derinel <derinel@google.com>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1451013}