feat: Added option to set callbackUrl as external

[zoey-kaiser]

Closes #553

You can now direct to an external url in the signin, signout and session methods:

signIn({ username, password }, { callbackUrl: 'https://sidebase.io', external: true })

Note: If this is set to true, then a hard page refresh will be done, therefore only set external to true, if you are really redirecting to a page outside of your application.

Checklist:

• issue number linked above after pound (#)
  • replace "Closes " with "Contributes to" or other if this PR does not close the issue
• manually checked my feature / checking not applicable
• wrote tests / testing not applicable
• attached screenshots / screenshot not applicable

[zoey-kaiser]

CC @IsraelOrtuno and @Jeroen-Cox as this is a follow up and change of how I implemented your solution in #429

[IsraelOrtuno]

I think this is good for now! 🎉