Allow string inputs for Long Scalar type

[jakobmerrild]

Solves: #1162

Let me know if this is really something that should be solved in sangria-circe instead?

[yanns]

There's a compilation error with scala 2.12:

 [error] /home/runner/work/sangria/sangria/modules/core/src/main/scala/sangria/schema/package.scala:50:27: value toLongOption is not a member of String
[error] Error occurred in an application involving default arguments.
[error]       case s: String if s.toLongOption.isDefined => Right(s.toLong)
[error]                           ^
[error] /home/runner/work/sangria/sangria/modules/core/src/main/scala/sangria/schema/package.scala:57:48: value toLongOption is not a member of String
[error] Error occurred in an application involving default arguments.
[error]       case ast.StringValue(s, _, _, _, _) if s.toLongOption.isDefined => Right(s.toLong)
[error]                                                ^

[jakobmerrild]

I tried to create a specification for a custom Long scalar: graphql/graphql-scalars#26