feat(GraphQLArgument): introduce optional arguments

[mkalen]

Extend GraphQLArgument annotation class with field "optional" (default to false for backward compatibility)

Proposal for handling of API with mutually exclusive arguments using annotations. See test cases in GraphQLRequestEntityTest.

Closes #56

[coveralls]

Pull Request Test Coverage Report for Build 81

• 37 of 38 (97.37%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 98.812%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
nodes/src/main/java/io/aexp/nodes/graphql/Parameter.java	8	9	88.89%
<!--	Total:	37	38	97.37%	-->

Totals
Change from base Build 78:	0.3%
Covered Lines:	499
Relevant Lines:	505

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 82

• 38 of 38 (100.0%) changed or added relevant lines in 6 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 99.01%

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Totals
Change from base Build 78:	0.5%
Covered Lines:	500
Relevant Lines:	505

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💛 - Coveralls