Enhancement request: Add support for Redis commands PING, HEXISTS

[scwagner]

ArcadeDB Version: v21.11.1-SNAPSHOT (build edfa409/1636409322106/main)

JDK Version: openjdk version "11.0.13" 2021-10-19

OS: Alpine Linux v3.14.2

Expected behavior

Calling the Redis functions of PING and HEXISTS to allow for more robust client integration.

Actual behavior

Calling either of these commands returns Command not found

Steps to reproduce

Using redis-cli, attempt to invoke either PING or HEXISTS commands against the server running with Redis compatibility plugin enabled:

127.0.0.1:6379> PING
(error) Command not found
127.0.0.1:6379> HEXISTS MyDatabase.Account[id] 123
(error) Command not found

[lvca]

Implemented.