feat: add highest_level= parameter to Actions class

[rich-iannone]

When defining actions for thresholds, the case may be that:

1. three thresholds are set
2. three actions are set (only per threshold level)
3. all three thresholds are exceeded and all three actions are triggered/executed

Having all three actions fire can result in undesired redundancy (i.e., over-signalling failure) and it's likely more often the case you'll want only the action from the most severe threshold that was exceeded.

This PR solves this by adding the highest_level= parameter to Actions class. It's set to True by default so the less common scenario (the one above) can be accessed with highest_level=False.

Here's an example of its use:

import pointblank as pb

(
     Validate(
         data=pb.load_dataset(dataset="small_table"),
         thresholds=pb.Thresholds(warning=1, error=2, critical=3),
     )
     .col_vals_gt(
         columns="d",
         value=10000,
         actions=pb.Actions(
             warning="W_local", error="E_local", critical="C_local", highest_only=True
         ),
     )
     .interrogate()
)

C_local

With highest_only=False we'd get this in the console:

C_local
E_local
W_local

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.03%. Comparing base (b2a910e) to head (b883922).

Files with missing lines	Patch %	Lines
pointblank/validate.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #124      +/-   ##
==========================================
- Coverage   99.05%   99.03%   -0.03%     
==========================================
  Files          17       17              
  Lines        3603     3610       +7     
==========================================
+ Hits         3569     3575       +6     
- Misses         34       35       +1     

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