envoy redis_proxy can‘t support RESP3 HELLO CMD

*Title*: *envoy redis_proxy can‘t support RESP3 HELLO CMD*

*Description*:
>I use a RESP3 redis client(lettuce) to connect envoy redis proxy,I found :
1) if envoy config downstream_auth_password, the use lettuce connect, envoy will return `-NOAUTH Authentication required.`

2) if envoy not config downstream_auth_password,then use lettuce connect, envoy will return `-unsupported command 'HELLO'`

If I use a RESP3 redis client(lettuce) to connect redis directly , there is no this problem.I think if envoy don't support HELLO，it should return `-ERR unknown command 'HELLO' ` like redis server return , so redis client lettuce can siwtch to RESP2.

*Repro steps*:
1) use https://github.com/lettuce-io/lettuce-core redis client (java) to connect envoy
2) or use telnet like this
[root@hadoop-master envoy]# telnet 127.0.0.1 63799
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
*5
$5
HELLO
$1
3
$4
AUTH
$7
default
$8
foobared

*Config*:
[envoy-redis-proxy-HELLO.yml.txt](https://github.com/envoyproxy/envoy/files/6920627/envoy-redis-proxy-HELLO.yml.txt)