Add support for `COPY` via `RedisOperations` and `ReactiveRedisOperations`

[mp911de]

COPY is a variant of MOVE that retains the key in the source database/key.

See also https://redis.io/commands/copy

[ihaohong]

hi, I am haohong, I write some code to support COPY command. This is my first-time to contribute to open source project.
I am a fish, aha. if I ignored some checklist, could you please help me to point out, And I will continue to fix it.

Thank you~

[mp911de]

Thank you for your contribution. That's merged and polished now.