Add NSCopying support to ART*ChannelOptions

Will use in an upcoming commit, but it's also handy for users of the SDK
(e.g. I had already found myself wanting it in Chat, when wishing to
create a modified version of an options object recevied as an argument).

TODO test

Source/ARTChannelOptions.m

@@ -17,6 +17,17 @@ - (instancetype)initWithCipherKey:(id<ARTCipherKeyCompatible>)key {
     return [self initWithCipher:@{@"key": key}];
 }
 
+- (id)copyWithZone:(NSZone *)zone {
+    ARTChannelOptions *copied = [[[self class] alloc] init];
+
+    // The _frozen flag prevents the instance we were copying from being mutated, but we don't yet want to prevent the new instance from being mutated
+    copied->_frozen = NO;
+
+    copied->_cipher = _cipher;
+
+    return self;
+}
+
 - (ARTCipherParams *)cipher {
     return _cipher;
 }

Source/ARTRealtimeChannelOptions.m

@@ -21,6 +21,16 @@ - (instancetype)initWithCipher:(id<ARTCipherParamsCompatible>)cipherParams {
     return self;
 }
 
+- (id)copyWithZone:(NSZone *)zone {
+    ARTRealtimeChannelOptions *copied = [super copy];
+
+    copied->_params = _params;
+    copied->_modes = _modes;
+    copied->_attachOnSubscribe = _attachOnSubscribe;
+
+    return copied;
+}
+
 - (NSStringDictionary *)params {
     return _params;
 }

Source/include/Ably/ARTChannelOptions.h

@@ -8,7 +8,7 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Passes additional properties to an `ARTRestChannel` object, such as encryption.
  */
-@interface ARTChannelOptions : NSObject
+@interface ARTChannelOptions : NSObject <NSCopying>
 
 /**
  * Requests encryption for this channel when not `nil`, and specifies encryption-related parameters (such as algorithm, chaining mode, key length and key). See [an example](https://ably.com/docs/realtime/encryption#getting-started).