[QuickStart] Add ability to request a close through Connection

Add the ability to manually request a close on Connection, threading
down a reason for the closure through all handlers. This will be used
to close the connection on timeout, deserialization failures, etc.

Test: Added unit tests in connection_unittest.cc
Bug: b/234655072
Change-Id: I5612fd2757c430b97494cf541308efc818c2d304
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4454929
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Brittany Hartmire <bhartmire@google.com>
Commit-Queue: Joe Antonetti <joeantonetti@google.com>
Reviewed-by: Curt Clemens <cclem@google.com>
Cr-Commit-Position: refs/heads/main@{#1135497}