sumdb: client and server protocol for checksum database

Copied from golang.org/x/exp/sumdb/internal/sumweb
with types cleaned up:

	Conn -> Client
	Client -> ClientOps
	Handler -> Server
	Server -> ServerOps

For golang/go#31761.

Change-Id: If0e004e6c9cab69c82de428810d67aba074aa843
Reviewed-on: https://go-review.googlesource.com/c/mod/+/176466
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
5 files changed
tree: e31d1f26ee276e081dc2fb4a16882d429cfbf4be
  1. LICENSE
  2. PATENTS
  3. README
  4. codereview.cfg
  5. go.mod
  6. go.sum
  7. module/
  8. semver/
  9. sumdb/