DX-1382: Generic EXEC command

[fahreddinozcan]

This PR adds a new method to execute arbitrary Redis commands. This command can be used where a command is defined on the Redis server, but not yet implemented on SDK. (we still work hard to remain compatible with the server). Here's an example use:

await redis.exec<number|null>(["MEMORY", "USAGE", dataKey])
redis.exec(["GET", "FOO"])

[linear]

DX-1382 Add some missing commands and a "exec" command to redis-js