Support for server plaintext upgrade

Looks like this is one of the supported protocol negotiators for clients but not for servers.

I've been trying to set up plaintext GRPC endpoints behind Amazon's new ALB (Application Load Balancer) and it looks like they require an upgradable connection on the backend (sadly, there is no way to configured ALB to assume HTTP/2).

I can try implementing a ProtocolNegotiator myself using Http2ServerUpgradeCodec but wanted to make sure this was feasible first.

Start by comparing the existing client protocol negotiator support and then inspect Http2ServerUpgradeCodec, both named in the issue. Determine whether a server-side ProtocolNegotiator can support plaintext upgrades for gRPC behind an Amazon ALB; done means that server configuration accepts the required upgrade path.

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