Expose a useNonce parameter on initWithConfiguration to make nonce nullable.

[FelipeBuiles]

Implement the solution described in #282

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@@            Coverage Diff            @@
##             master     #283   +/-   ##
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  Coverage          ?   73.52%           
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  Files             ?       58           
  Lines             ?     4933           
  Branches          ?        0           
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  Hits              ?     3627           
  Misses            ?     1306           
  Partials          ?        0

Impacted Files	Coverage Δ
Source/OIDAuthorizationRequest.h	100% <ø> (ø)
Source/OIDAuthorizationRequest.m	83.61% <0%> (ø)

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[WilliamDenniss]

I don't think this is needed. We already support not sending nonce. You can pass nil into the designated initializer:

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;

Nonce is not added if nil:

AppAuth-iOS/Source/OIDAuthorizationRequest.m

Lines 319 to 321 in b4ca39a

	if (_nonce) {
	[query addParameter:kNonceKey value:_nonce];
	}