UploadPack: support custom packfile-to-URI mapping

UploadPack: support custom packfile-to-URI mapping

Teach UploadPack to take a provider of URIs corresponding to cached
packs. When fetching, if the client supports the packfile-uri feature,
and if such a cached pack were to be streamed, instead send the
corresponding URI.

This packfile-uri feature is implemented in the jt/fetch-cdn-offload
branch of Git. There is interest in this feature [1], but it is not yet
merged.

[1] https://public-inbox.org/git/cover.1552073690.git.jonathantanmy@google.com/

Change-Id: I9a32dae131c9c56ad2ff4a8a9638ae3b5e44dc15
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>