[3.x] Replace DTO's with Fluent objects.

[r-kujawa]

What does this PR do? 🤖

• Replaces the Provider::class DTO with Provider::class Fluent object.
• Replaces the Account::class DTO with Account::class Fluent object.
• 💅🏼 Some additional quality of code improvements.

Does this relate to any issue? 🔗

Closes #72

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 92.14%. Comparing base (ebeee27) to head (f9b9856).

Files	Patch %	Lines
src/Fluent/Provider.php	80.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #80      +/-   ##
============================================
- Coverage     93.09%   92.14%   -0.95%     
+ Complexity      190      189       -1     
============================================
  Files            22       22              
  Lines           666      662       -4     
============================================
- Hits            620      610      -10     
- Misses           46       52       +6     

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