Add PAR (Pushed Authorization Request) support

[subhankarmaiti]

adds support for Pushed Authorization Request (PAR) flows where the backend-for-frontend (BFF) handles the /par and /token endpoints while the SDK manages the browser-based authorization.

Usage

// Callback style
WebAuthProvider.authorizeWithRequestUri(account)
    .start(context, requestUri, object : Callback<AuthorizationCode, AuthenticationException> {
        override fun onSuccess(result: AuthorizationCode) {
            // Exchange result.code for tokens via BFF
        }
        override fun onFailure(error: AuthenticationException) {
            // Handle error
        }
    })

// Coroutines
val authCode = WebAuthProvider.authorizeWithRequestUri(account)
    .await(context, requestUri)
// Exchange authCode.code for tokens via BFF