[client] Pass the login hint to GetOAuthFlow at construction

GetOAuthFlow was the only flow factory without a hint parameter, which
forced its callers to apply the hint afterwards through a local setter
interface and a type assertion. Give it the same constructor-style hint
as NewOAuthFlow and set the hint on the concrete flows before they are
handed out as the interface, so a flow is always complete when built
and the caller-side ordering constraint disappears.

An empty hint is a valid value meaning the IdP chooses the account, so
the flows set it unconditionally.

client/ios/NetBirdSDK/login.go

@@ -323,7 +323,7 @@ func (a *Auth) login(urlOpener URLOpener, forceDeviceAuth bool, deviceName strin
 const authInfoRequestTimeout = 30 * time.Second
 
 func (a *Auth) foregroundGetTokenInfo(authClient *auth.Auth, urlOpener URLOpener, forceDeviceAuth bool) (*auth.TokenInfo, error) {
-	oAuthFlow, err := authClient.GetOAuthFlow(a.ctx, forceDeviceAuth)
+	oAuthFlow, err := authClient.GetOAuthFlow(a.ctx, forceDeviceAuth, "")
 	if err != nil {
 		return nil, fmt.Errorf("failed to get OAuth flow: %v", err)
 	}