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.

Comienza comparando la compatibilidad existente con el negociador de protocolos del cliente y, después, inspecciona Http2ServerUpgradeCodec, ambos mencionados en el issue. Determina si un ProtocolNegotiator del lado del servidor puede admitir actualizaciones en texto plano para gRPC detrás de un Amazon ALB; se considera terminado cuando la configuración del servidor acepta la ruta de actualización requerida.

Escrito por el modelo de indexación a partir del texto del issue.