Expose initialisers of GraphQLInputObjectType and InputObjectField as…

[sportlabsMike]

… public

So that GraphQLInputObjectType objects can be constructed by users of the framework.

[codecov-io]

Codecov Report

Merging #23 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage    80.2%   80.16%   -0.04%     
==========================================
  Files          78       78              
  Lines        9239     9243       +4     
==========================================
  Hits         7410     7410              
- Misses       1829     1833       +4

Impacted Files	Coverage Δ
Sources/GraphQL/Type/Definition.swift	48.88% <0%> (-0.45%)	⬇️

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