[DE-666] Cluster Rebalance

[apetenchea]

This PR adds the following functionality to the Cluster class:

• calculate_imbalance (GET /_admin/cluster/rebalance) - used for getting the current cluster imbalance
• rebalance (PUT /_admin/cluster/rebalance) - used for rebalancing the shard distribution (calculates and then executes a rebalancing plan)
• calculate_rebalance_plan (POST /_admin/cluster/rebalance) - calculates (but does not execute) a rebalancing plan
• execute_rebalancing_plan (POST /_admin/cluster/rebalance/execute) - executes an already calculated rebalancing plan

A small documentation example has been added, together with the unit tests.

For reference, see the Rebalancing Section of the official documentation.

[codecov-commenter]

Codecov Report

Merging #287 (3a7d0df) into main (744a6e9) will decrease coverage by 0.41%.
Report is 17 commits behind head on main.
The diff coverage is 94.65%.

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@@ Coverage Diff @@ ## main #287 +/- ## ========================================== - Coverage 99.01% 98.60% -0.41%  ========================================== Files 26 26 Lines 3958 4014 +56 ========================================== + Hits 3919 3958 +39  - Misses 39 56 +17 

Files Changed	Coverage Δ
arango/client.py	98.41% <ø> (ø)
arango/http.py	100.00% <ø> (ø)
arango/collection.py	97.41% <92.92%> (-1.09%)	⬇️
arango/cluster.py	100.00% <100.00%> (ø)
arango/database.py	99.73% <100.00%> (-0.13%)	⬇️
arango/exceptions.py	100.00% <100.00%> (ø)
arango/request.py	94.44% <100.00%> (ø)
arango/utils.py	87.50% <100.00%> (-12.50%)	⬇️

... and 2 files with indirect coverage changes