Added RedirectUrl into swagger.go to enable custom redirect URL

[leungas]

Describe the PR Currently echo-swagger assumes the Redirect URL used in OAuth2 process flow is at the root. Thus in the case of when Swagger UI is not root, the redirection would not work and result in 404 Not Found returned. This PR will enable a new parameter RedirectUrl parameter and can be used along with the EchoWrapHandler so it can overwritten if necessary.

Relation issue None - new feature

Additional context If the RedirectURL parameter is not set, it will simply take the existing echo-swagger functions.

[leungas]

Just update the changes as mentioned above.

[ubogdan]

Would you please fix the code and resolve conflicts to go forward with it?