Feature: add \shards command to track relocations

[mannreis]

Summary of the changes / Why this is an improvement

As requested in #493 this PR aims adds a new command \shards to conveniently query for shards undergoing relocation. It computes the the aggregated progress per table based on the sum of the sizes of relocating shards divided by the total size of the table shards.

Optionally, state and relocating can be provided as an argument to respectively:

• get a short summary of shards aggregated by state
• view details of which ones are currently relocating

Integration testing

I resorted to docker compose and scripting to move shards withing a cluster in order to test the what's implemented here. In order integrate that type of testing it in this code base, I spent sometime looking at cratedb_toolkit.testing.testcontainers.cratedb but I believe it would require another PR. For now, I resort to single-node integration testing.

Checklist

• Fixes Add internal command to watch shard relocations progresses #493
• Unit testing
• Integration testing
• Command docs
• Update CHANGES.txt

[seut]

I've added some suggestions related to the used statements, otherwise looks fine.
I had a bit in mind that having some ongoing progress visible with some kind of progress bar, which one could get out e.g. by ctrl-c would be awesome, but also probably better to work on this as a followup, now that you've done the ground work :)

About real integration testing: Right, this would require at least 2 nodes to make it work. Utilizing the testcontainers to make this work would be really great, but agree, lets look into this as followup as well.

[seut]

@mannreis any update on this, do you need any further help to continue?

[mannreis]

Your suggestions were already a great help! I'll apply them early next week. They seem easy enough to have been applied already.

Regarding your original idea of an interactive progress bar. I saw that postgres cli had a \watch command that seemed useful and could be used for queries but I guess other commands as well.