[1.x] Add config column to merchants & providers tables.

[r-kujawa]

What does this PR do? 🤖

• Adds config json column to the merchants & providers tables.
• Updates models to cast json to array.

Any background context you would like to provide? 🚧

With the config driver you may specify additional configurations under each serviceable. Now you may do the same using the database driver, via the config column.

Does this relate to any issue? 🔗

Closes #34

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.58%. Comparing base (baf4e15) to head (b8447a7).

Additional details and impacted files

@@            Coverage Diff            @@
##                1.x      #58   +/-   ##
=========================================
  Coverage     92.58%   92.58%           
  Complexity      186      186           
=========================================
  Files            21       21           
  Lines           647      647           
=========================================
  Hits            599      599           
  Misses           48       48           

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