Allow mass_assignment option for belongs_to relation

[dukeraphaelng]

Description

• This PR allows mass_assign option for belongs_to relation

Motivation and Context

• Currently mass_assign in the JSONDeserialize module only supports column but not the belongs_to relation which would be essential if belongs_to foreign_key is nilable.
• Please note, the example data used in the spec for this feature is dependent on this PR Fix JSON::Deserialize not working with column_name field #203

How Has This Been Tested?

• All specs passed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

• Manual of usage of the new feature.

Checklist:

• My code follows the code style of this project. bin/ameba ran without alert.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.