design/25530-sumdb: change /sumdb/supported to /sumdb/<sumdb-name>/supported
design/25530-sumdb: change /sumdb/supported to /sumdb/<sumdb-name>/supported

Telling the proxy explicitly which checksum database we are asking about
will make it possible, if there are ever multiple checksum databases,
to access one database through one proxy and another through another proxy.
As written before, that would be imposible.

The old rationale for a single /sumdb/supported was to make it
possible for corp proxies to say "I support all databases" and thereby
intercept all possible database traffic. Those proxies can still do that
by responding 200 to /sumdb/.*/supported.

Change-Id: I8ce1a75d03e0d9dec8907fd3869cc8a83f82f831
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/173946
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

golang/proposal,golang/proposal