Make nonce optional for OAuth servers that don't support it.

[FelipeBuiles]

AppAuth-iOS worked correctly with AWS Cognito up until its 0.93 release that fixed #4. The issue lies in the inabilty of Cognito to handle the nonce parameter (mentioned in their forums here).
There is a comment on the OIDAuthorizationRequest.h file that mentions something about it being nullable:

    @param nonce String value used to associate a Client session with an ID Token. Can be set to nil
        if not using OpenID Connect, although pure OAuth servers should ignore params they don't
        understand anyway.

The thing is, my team is not using AppAuth-iOS directly but rather trough a wrapper since our project is built with React Native, and we need a way to expose this functionality to that wrapper.

I've opened a PR that implements our fix to this, its also linked to another PR on the wrapper repo.

[WilliamDenniss]

The right approach here (to avoid sending nonce to a non-compliant provider) is to use the designated initializer of OIDAuthorizationRequest and pass nil for the nonce.

AppAuth-iOS/Source/OIDAuthorizationRequest.h

Lines 183 to 216 in b4ca39a

	/*! @brief Designated initializer.
	@param configuration The service's configuration.
	@param clientID The client identifier.
	@param scope A scope string per the OAuth2 spec (a space-delimited set of scopes).
	@param redirectURL The client's redirect URI.
	@param responseType The expected response type.
	@param state An opaque value used by the client to maintain state between the request and
	callback.
	@param nonce String value used to associate a Client session with an ID Token. Can be set to nil
	if not using OpenID Connect, although pure OAuth servers should ignore params they don't
	understand anyway.
	@param codeVerifier The PKCE code verifier. See @c OIDAuthorizationRequest.generateCodeVerifier.
	@param codeChallenge The PKCE code challenge, calculated from the code verifier such as with
	@c OIDAuthorizationRequest.codeChallengeS256ForVerifier:.
	@param codeChallengeMethod The PKCE code challenge method.
	::OIDOAuthorizationRequestCodeChallengeMethodS256 when
	@c OIDAuthorizationRequest.codeChallengeS256ForVerifier: is used to create the code
	challenge.
	@param additionalParameters The client's additional authorization parameters.
	*/
	- (instancetype)
	initWithConfiguration:(OIDServiceConfiguration *)configuration
	clientId:(NSString *)clientID
	clientSecret:(nullable NSString *)clientSecret
	scope:(nullable NSString *)scope
	redirectURL:(nullable NSURL *)redirectURL
	responseType:(NSString *)responseType
	state:(nullable NSString *)state
	nonce:(nullable NSString *)nonce
	codeVerifier:(nullable NSString *)codeVerifier
	codeChallenge:(nullable NSString *)codeChallenge
	codeChallengeMethod:(nullable NSString *)codeChallengeMethod
	additionalParameters:(nullable NSDictionary<NSString *, NSString *> *)additionalParameters
	NS_DESIGNATED_INITIALIZER;

[FelipeBuiles]

You're completely right, I'll update my PR on the react-native-app-auth repo to use the correct initializer. Thanks for the guidance.