Expose more advanced OIDAuthorizationRequest configuration options.

The current interface allows developers to configure their authorization requests on a very basic level with just `clientId`, `audience`, `hosted domain` and `login_hint`, leaving more advanced parameters outside of their control. However, `OIDAuthorizationRequest` exposes a more complex initializer than [the one currently in use](https://github.com/openid/AppAuth-iOS/blob/master/Source/AppAuthCore/OIDAuthorizationRequest.m#L125):

```
- (instancetype)initWithConfiguration:(OIDServiceConfiguration *)configuration
clientId:(NSString *)clientID
clientSecret:(nullable NSString *)clientSecret
scope:(nullable NSString *)scope
redirectURL:(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 *)additionalParameters
```
which (in theory) means, that the limitation is only in `GIDConfiguration`.

Since we're talking about a piece of software made by developers for developers, I believe making it more flexible for non-standard and advanced use cases can only improve it.