Support for required and partial

[shmax]

Adding two new features:

1. required this is a convenience property on any field definition. When set to true and the value is not passed-in, an error will automatically be generated for you in this form of [1, "<name> is required"]. Alternatively, you can set required to the error you would like to get back when the value is not provided (eg. required: [1, 'my own custom error message']).
2. 'partial'. When true, we will only validate any values passed in. When false, we will validate any required fields on the InputObject even when not provided.

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 95.78%. Comparing base (3d6b1ce) to head (8ae1fcf).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/Type/Definition/ValidatedFieldDefinition.php	72.22%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   97.60%   95.78%   -1.83%     
==========================================
  Files           2        2              
  Lines         251      261      +10     
==========================================
+ Hits          245      250       +5     
- Misses          6       11       +5     

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