tunnel: return tunnel token, fix uri

[bradleyglover]

Description

Update TunnelToken to return the result as a string. Also fixes the uri to start with /

Downstream PR here: cloudflare/terraform-provider-cloudflare#1590 (to be updated)

Has your change been tested?

Tested locally with changes to the terraform provider (see above PR) to verify token result was retrievable and properly set in state.

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[jacobbednarz]

added a simple test to cover the change here (which would have caught the issue initially)