Add types to PurgatoryPropertyAccessor

[Brajk19]

since version 2 will not support SF5, we can add types

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.33%. Comparing base (d8a2471) to head (670863c).
⚠️ Report is 1 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...vider/PropertyAccess/PurgatoryPropertyAccessor.php	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##                2.x     #132   +/-   ##
=========================================
  Coverage     97.33%   97.33%           
  Complexity      509      509           
=========================================
  Files            69       69           
  Lines          1426     1426           
=========================================
  Hits           1388     1388           
  Misses           38       38           

Flag	Coverage Δ
8.1	97.33% <50.00%> (ø)
8.2	97.26% <50.00%> (ø)
8.3	97.05% <50.00%> (ø)
8.4	97.05% <50.00%> (ø)
8.5	97.05% <50.00%> (ø)

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