Add "many" version of API calls for UpdateMagicTransitGRETunnels, CreateMagicTransitStaticRoutes & UpdateMagicTransitStaticRoutes

[ivan-section-io]

Description

During work automating Magic Transit GRE Tunnel & Magic Transit Static Route setup, I discovered missing go wrappers that accepted multiple items.

The "many" versions I found absent were:
UpdateMagicTransitGRETunnels https://api.cloudflare.com/#magic-gre-tunnels-update-multiple-gre-tunnels
CreateMagicTransitStaticRoutes https://api.cloudflare.com/#magic-static-routes-create-routes
UpdateMagicTransitStaticRoutes https://api.cloudflare.com/#magic-static-routes-update-many-routes
DeleteMagicTransitStaticRoutes https://api.cloudflare.com/#magic-static-routes-delete-many-routes

Without these, iterating over larger sets with the single item calls is painfully slow.

Has your change been tested?

Automated tests mimicking other multi-item versions have been added.
These added calls are currently being used in our internal setup tool.

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.