Enumerate service modes with explicit values

[dhens]

Description

Our current type for the Mode field on the ServiceModeV2 struct is a string. This allows for arbitrary strings, which are not supported service modes.

The Mode field now uses a new ServiceMode string type that has constant values that can be assigned. These values are all of the existing service modes.

Has your change been tested?

The underlying Mode type has not changed, so we don't need to augment the current tests.

Types of changes

• New feature (non-breaking change which adds functionality)

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.

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v0.64.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!