Add API for demoting group members (#445)

apigrpc/apigrpc.pb.go

apigrpc/apigrpc.pb.gw.go

apigrpc/apigrpc.proto

apigrpc/apigrpc.swagger.json

console/console.pb.go

console/console.swagger.json

go.mod

go.sum

server/api_group.go

server/core_group.go

server/pipeline_channel.go

server/runtime.go

server/runtime_go.go

server/runtime_go_logger.go

server/runtime_go_logger_test.go

server/runtime_lua.go

vendor/github.com/heroiclabs/nakama-common/api/api.pb.go

vendor/github.com/heroiclabs/nakama-common/api/api.proto

...or/github.com/heroiclabs/nakama-common/runtime/runtime.go

vendor/go.uber.org/zap/zaptest/observer/logged_entry.go