Move EC2 array coder from Soto (#536)

* Move EC2ArrayCoder from Soto

* Comments

Sources/SotoCore/Encoder/CodableProperties/EC2ArrayCoder.swift

@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Soto for AWS open source project
+//
+// Copyright (c) 2023 the Soto project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Soto project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// Coder for encoding/decoding Arrays for EC2 service. This is extended to support encoding and
+/// decoding based on whether `Element` is `Encodable` or `Decodable`.
+///
+/// EC2 requires a special case array encoder as it flattens arrays on encode, while still expecting
+/// them to have array elements on decode.
+public struct EC2ArrayCoder<Properties: ArrayCoderProperties, Element: _SotoSendable>: CustomCoder {
+    public typealias CodableValue = [Element]
+}
+
+/// Standard EC2 Array Coder with element name "member"
+public typealias EC2StandardArrayCoder<Element> = EC2ArrayCoder<StandardArrayCoderProperties, Element>
+
+/// CodingKey used by Encoder property wrappers
+private struct _EncodingWrapperKey: CodingKey {
+    public var stringValue: String
+    public var intValue: Int?
+
+    public init?(stringValue: String) {
+        self.stringValue = stringValue
+        self.intValue = nil
+    }
+
+    public init?(intValue: Int) {
+        self.stringValue = "\(intValue)"
+        self.intValue = intValue
+    }
+
+    public init(stringValue: String, intValue: Int?) {
+        self.stringValue = stringValue
+        self.intValue = intValue
+    }
+}
+
+/// extend to support decoding
+extension EC2ArrayCoder: CustomDecoder where Element: Decodable {
+    public static func decode(from decoder: Decoder) throws -> CodableValue {
+        let topLevelContainer = try decoder.container(keyedBy: _EncodingWrapperKey.self)
+        var values: [Element] = []
+        let memberKey = _EncodingWrapperKey(stringValue: Properties.member, intValue: nil)
+        guard topLevelContainer.contains(memberKey) else { return values }
+
+        var container = try topLevelContainer.nestedUnkeyedContainer(forKey: memberKey)
+        while !container.isAtEnd {
+            values.append(try container.decode(Element.self))
+        }
+        return values
+    }
+}
+
+/// extend to support encoding (flatten the array)
+extension EC2ArrayCoder: CustomEncoder where Element: Encodable {
+    public static func encode(value: CodableValue, to encoder: Encoder) throws {
+        try value.encode(to: encoder)
+    }
+}