TDigest: Implement `TDIGEST.MERGE` command

[LindaSummer]

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• [x] I had searched in the issues and found no similar issues.

Motivation

Refer: #2558 We should implement TDIGEST.MERGE command.

Redis command reference: https://redis.io/docs/latest/commands/tdigest.merge/

Solution

Depends on #2791 #2792 #2794 .

Add go integration tests for it.

Are you willing to submit a PR?

• [x] I'm willing to submit a PR!