Implement validator using Visitor pattern

[mperor]

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[github-actions]

Test Results

234 tests  +2   231 ✅ +2   5s ⏱️ ±0s
 73 suites +1     3 💤 ±0 
 73 files   +1     0 ❌ ±0 

Results for commit 7866275. ± Comparison against base commit ab42078.

This pull request removes 2 and adds 4 tests. Note that renamed tests count towards both.

pl.mperor.lab.java.design.pattern.behavioral.visitor.CustomerVisitorTest ‑ testDiscountLevelForDifferentTypeCustomer()
pl.mperor.lab.java.design.pattern.behavioral.visitor.CustomerVisitorTest ‑ testGenerateInvitationLetterForDifferentTypeCustomer()

pl.mperor.lab.java.design.pattern.behavioral.visitor.customer.CustomerVisitorTest ‑ testDiscountLevelForDifferentTypeCustomer()
pl.mperor.lab.java.design.pattern.behavioral.visitor.customer.CustomerVisitorTest ‑ testGenerateInvitationLetterForDifferentTypeCustomer()
pl.mperor.lab.java.design.pattern.behavioral.visitor.validator.ValidatorVisitorTest ‑ testValidatePasswordAndEmailWithExceptionAggregation()
pl.mperor.lab.java.design.pattern.behavioral.visitor.validator.ValidatorVisitorTest ‑ testValidatePasswordAndEmailWithExceptionThrowing()

This pull request skips 1 and un-skips 1 tests.

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testLazyInitialized()

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testThreadSafe()