Allow multiple @link schema extensions

[BoD]

Adding multiple @link extensions was failing because the definition of @link wasn't known, so couldn't check if it's repeatable here.

[svc-apollo-docs]

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[martinbonnin]

Can we have getImports return its own foreign schema for @link? @link is a very special case and I'd rather keep everything contained in getImports()

[martinbonnin]

Something along these lines:

      if (gqlDirective.name == "link") {
        val linkForeignSchema = ForeignSchema("link", "v1.0", """
              directive @link(
                url: String!,  
                as: String,
                import: [Import],
                for: Purpose)
                repeatable on SCHEMA

              scalar Import

              enum Purpose {
                SECURITY
                EXECUTION
              }

            """.trimIndent().toGQLDocument().definitions
        )
        imports.add(
            Import(linkForeignSchema, linkForeignSchema.definitions, mapOf("link" to "link"))
        )