Limit code_challenge to shouldExchangeAuthCode only

[mohssenfathi]

Description

This PR fixes a bug where we send code_challenge in the authorize request when we shouldn't be. We should only be sending a codeChallenge to the authorize endpoint in cases where we want to exchange auth code for auth token on device.

Changes

• Made AuthorizeRequest's codeChallenge property optional
• Changed AuthorizeRequest's payload to only set code_challenge_method if codeChallenge is non-nil
• From AuthorizationCodeAuthProvider, only send pace.codeChallenge if shouldExchangeAuthCode is true

Testing

• Added unit tests to verify code_challenge and code_challenge_method are not being sent for both inApp and native login destinations