QuickStart: Use Base64 type for FIDO credential id

Use Base64 as the type of FIDO credential id instead of std::string.
This will help us prevent encoding bugs. It is impossible to guess the
encoding of a std::string data, and can lead to double-encoding or
no-encoding bugs.

Additionally, use Base64 instead of Base64Url as the underlying encoding
(see https://crrev.com/c/5013510). This follows the encoding guideline
at https://protobuf.dev/programming-guides/proto3/#json. GoogleFE /
OnePlatform is lenient in allowing Base64Url representation of byte
array proto fields, but the canonical representation is still Base64
(not Base64Url).

Bug: b/289996257, b/289996415
Test: unit_tests --gtest_filter="*ConnectionTest*"
Test: unit_tests --gtest_filter="*SecondDeviceAuthBrokerTest*"
Change-Id: If75286396bc932190bcbe80aed4d924225613571
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5012811
Reviewed-by: Renato Silva <rrsilva@google.com>
Reviewed-by: Michael Hansen <hansenmichael@google.com>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1223578}