Upgrade to GraphQL.NET v4 + support for NRT

[Shane32]

• Works with GraphQL.NET 4.0.2+
• Adds support for nullable reference types.
• [Required] works same as before
• Added [Optional]
• If no NRT annotations for the type, assumes that objects are nullable (and value types are not)
• [Required] and [Optional] annotations override all else
• Added full suite of testing

[github-actions]

Coverage Report

Totals
Statements:	91.72% ( 399 / 435 )
Methods:	78.05% ( 32 / 41 )