planner: introduce SPOT VMs policy

[csegarragonz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 63.71429% with 127 lines in your changes are missing coverage. Please review.

Project coverage is 81.33%. Comparing base (35af765) to head (621fdfc).

Files	Patch %	Lines
src/planner/Planner.cpp	37.31%	84 Missing ⚠️
src/executor/Executor.cpp	0.00%	17 Missing ⚠️
src/scheduler/Scheduler.cpp	33.33%	14 Missing ⚠️
src/planner/PlannerEndpointHandler.cpp	85.36%	6 Missing ⚠️
src/batch-scheduler/SpotScheduler.cpp	96.15%	5 Missing ⚠️
include/faabric/util/func.h	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #433      +/-   ##
==========================================
- Coverage   82.25%   81.33%   -0.93%     
==========================================
  Files         116      117       +1     
  Lines        7901     8233     +332     
==========================================
+ Hits         6499     6696     +197     
- Misses       1402     1537     +135     

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