Support overload control

[tjoubert]

Added OverloadControlDatabase, enabling the client to react effectively to potential server overloads.

Requests go through the OverloadControlApiExecutor. For every API call, it allows clients to pass the x-arango-max-queue-time-seconds request header and also return the X-Arango-Queue-Time-Seconds response header if it exists.

Additional tests and documentation added ✔️

[joowani]

The approach makes sense. Regarding the name QueueTimeApiExecutor, perhaps we could rename it to something more generic so we can reuse to support other headers in the future (e.g. ApiExecutorV2)

[codecov-commenter]

Codecov Report

Merging #237 (8b09e07) into main (52648c7) will increase coverage by 0.01%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   98.57%   98.58%   +0.01%     
==========================================
  Files          26       26              
  Lines        3858     3899      +41     
==========================================
+ Hits         3803     3844      +41     
  Misses         55       55              

Impacted Files	Coverage Δ
arango/database.py	99.86% <100.00%> (+<0.01%)	⬆️
arango/exceptions.py	100.00% <100.00%> (ø)
arango/executor.py	99.47% <100.00%> (+0.07%)	⬆️