Make QuicSessionPool::SessionAttempt public

This CL is in preparation for supporting QUIC in HttpStreamPool. HttpStreamPool perfoms DNS resolution so it doesn't use
QuicSessionRequest. Instead, it directly requests QuicSessionPool to
attempt a session by providing a resolved IPEndPoint and metadata
(see the design doc [1] for the overall plan).

This CL moves QuicSessionPool::SessionAttempt out of QuicSessionPool
to make it public. This CL also adds a delegate interface to remove
the direct dependency on QuicSessionPool::Job from QuicSessionAttempt.

This CL has no behavior change.

[1] https://docs.google.com/document/d/10zr0g6E7RNUK224DTMC89gvZrwJtCIH4Lam-ZW5q0Kw/edit#heading=h.8fzco31ljqle

Bug: 346835898
Change-Id: I8e30bf9b4ba3ec5fc52a82b126fd81828b9aace5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5758665
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Ryan Hamilton <rch@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1337663}