Validator Tracing and TimeTriggeredValidator

[mikand]

In this PR we extend the internal sequential validation engine to return the sequence of states encountered by executing a sequential plan and we add support for validation (and tracing) of TimeTriggeredPlans

[karpase]

Commenting just to say I would like to see this implemented - I have some experiments this could really help with.

[Framba-Luca]

In pr #509 also happens a "transformation" of a temporal structure into a non-temporal one. After the merge of both there will most certainly be space for refactoring (that maybe can be considered also when designing the missing code).

[codecov-commenter]

Codecov Report

Attention: 36 lines in your changes are missing coverage. Please review.

Comparison is base (da8ab70) 84.96% compared to head (2567ee5) 84.92%.
Report is 16 commits behind head on master.

Files	Patch %	Lines
unified_planning/engines/plan_validator.py	83.80%	34 Missing ⚠️
unified_planning/test/test_plan_validator.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
- Coverage   84.96%   84.92%   -0.04%     
==========================================
  Files         200      200              
  Lines       26433    26611     +178     
==========================================
+ Hits        22458    22600     +142     
- Misses       3975     4011      +36     

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[Framba-Luca]

LGTM! 😃

I left 4 comments. 2 are minor code-level improvements, 1 I think it's a bug, the 4th one is just a note on the SequentialValidator supported_kind

[arbimo]

LGTM! I just added a small comment about an awkwardness in error handling.