Support grpc-web about connect-go

[akshayjshah]

One more caveat: the protocol document makes it pretty clear that the protocol follows the implementation, rather than the other way around. I can't tell how stable the protocol is, and whether we'll get ourselves into a bad semver situation by implementing it.

If we do implement it, I lean towards just putting it in and enabling it by default, like our current Twirp support. Unless there's some broader array of proto-over-HTTP dialects we see ourselves supporting in v1, a plugin system for this seems overly elaborate.