Add missing imports to fix Swift 6.1+ (#709)

### Motivation

Currently unit tests on Swift 6.1+ toolchains are failing due to a few
missing imports.

That these are coming up is good, that's why we've enabled
`MemberImportVisibility`.

### Modifications

Add the missing imports to stop relying on implicitly imported extension
methods.

### Result

Unit tests compile again on Swift 6.1+.

### Test Plan

Tested locally, will further verify CI is happy.

.github/workflows/pull_request.yml

@@ -21,7 +21,7 @@ jobs:
             linux_5_10_arguments_override: "--explicit-target-dependency-import-check error"
             linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error"
             linux_nightly_6_0_arguments_override: "--explicit-target-dependency-import-check error"
-            linux_nightly_main_enabled: false
+            linux_nightly_main_arguments_override: "--explicit-target-dependency-import-check error"
 
     integration-test:
         name: Integration test

Sources/_OpenAPIGeneratorCore/Extensions/SwiftStandardLibrary.swift

@@ -11,6 +11,8 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 //===----------------------------------------------------------------------===//
+import Foundation
+
 extension Int {
     /// Returns the digits for the number using the specified radix.
     /// - Parameter radix: The radix used to format the integer.

Sources/_OpenAPIGeneratorCore/Translator/CommonTypes/StructBlueprint.swift

@@ -12,6 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 import OpenAPIKit
+import Foundation
 
 /// A structure that contains the information about an OpenAPI object that is
 /// required to generate a matching Swift structure.

Sources/_OpenAPIGeneratorCore/Translator/Content/ContentInspector.swift

@@ -12,6 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 import OpenAPIKit
+import Foundation
 
 /// Utilities for asking questions about OpenAPI.Content
 extension FileTranslator {

Sources/_OpenAPIGeneratorCore/Translator/TypesTranslator/translateServersVariables.swift

@@ -12,6 +12,7 @@
 //
 //===----------------------------------------------------------------------===//
 import OpenAPIKit
+import Foundation
 
 /// Represents a server variable and the function of generation that should be applied.
 protocol ServerVariableGenerator {