Agent check gRPC version against server (#1653)

close #1114

As long as the `VersionResponse` type is not changed the check will
fail/pass gracefully

example output:
```
{"level":"error","error":"GRPC version mismatch","time":"2023-03-19T19:49:09+01:00","message":"Server version next-6923e7ab does report grpc version 2 but we only understand 1"}
GRPC version mismatch
```

cmd/agent/agent.go

@@ -1,3 +1,4 @@
+// Copyright 2023 Woodpecker Authors
 // Copyright 2018 Drone.IO Inc.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -17,6 +18,7 @@ package main
 import (
 	"context"
 	"crypto/tls"
+	"errors"
 	"net/http"
 	"os"
 	"runtime"
@@ -154,6 +156,21 @@ func loop(c *cli.Context) error {
 		sigterm.Set()
 	})
 
+	// check if grpc server version is compatible with agent
+	grpcServerVersion, err := client.Version(ctx)
+	if err != nil {
+		log.Error().Err(err).Msg("could not get grpc server version")
+		return err
+	}
+	if grpcServerVersion.GrpcVersion != agentRpc.ClientGrpcVersion {
+		err := errors.New("GRPC version mismatch")
+		log.Error().Err(err).Msgf("Server version %s does report grpc version %d but we only understand %d",
+			grpcServerVersion.ServerVersion,
+			grpcServerVersion.GrpcVersion,
+			agentRpc.ClientGrpcVersion)
+		return err
+	}
+
 	backendCtx := context.WithValue(ctx, types.CliContext, c)
 	backend.Init(backendCtx)