add swift5 option for generating frozen enums (#11013)

* add swift5 option for generating frozen enums

* use case unknownDefault to avoid conflicts

* update comments to reflect unknownDefault case

* set default values for unknown case to avoid conflict

* dont need vendor extensions to detect enum raw data type

* move CaseIterableDefaultsLast into models mustache template

* comment catch all case and add support for other types

* add frozen enums to ci pipeline

* remove extraneous edit to extensions template

* remove left over protocols files

* small comment and case adjustments

samples/client/petstore/swift5/asyncAwaitLibrary/PetstoreClient/Classes/OpenAPIs/Models.swift

@@ -14,6 +14,28 @@ extension JSONEncodable {
     func encodeToJSON() -> Any { self }
 }
 
+/// An enum where the last case value can be used as a default catch-all.
+protocol CaseIterableDefaultsLast: Decodable & CaseIterable & RawRepresentable
+where RawValue: Decodable, AllCases: BidirectionalCollection {}
+
+extension CaseIterableDefaultsLast {
+    /// Initializes an enum such that if a known raw value is found, then it is decoded.
+    /// Otherwise the last case is used.
+    /// - Parameter decoder: A decoder.
+    public init(from decoder: Decoder) throws {
+        if let value = try Self(rawValue: decoder.singleValueContainer().decode(RawValue.self)) {
+            self = value
+        } else if let lastValue = Self.allCases.last {
+            self = lastValue
+        } else {
+            throw DecodingError.valueNotFound(
+                Self.Type.self,
+                .init(codingPath: decoder.codingPath, debugDescription: "CaseIterableDefaultsLast")
+            )
+        }
+    }
+}
+
 public enum ErrorResponse: Error {
     case error(Int, Data?, URLResponse?, Error)
 }