Adds support for Magic Transit IPsec tunnels

[cehrig]

Description

Adds support for Magic Transit IPsec Tunnels.

API docs:
List Tunnels: https://api.cloudflare.com/#magic-ipsec-tunnels-list-ipsec-tunnels
Get Tunnel: https://api.cloudflare.com/#magic-ipsec-tunnels-ipsec-tunnel-details
Create Tunnel: https://api.cloudflare.com/#magic-ipsec-tunnels-create-ipsec-tunnels
Update Tunnel: https://api.cloudflare.com/#magic-ipsec-tunnels-update-ipsec-tunnel
Delete Tunnel: https://api.cloudflare.com/#magic-ipsec-tunnels-delete-ipsec-tunnel

Has your change been tested?

$ go test $(go list ./...) -v -run "^Test.*MagicTransitIPsecTunnel.*$"
=== RUN   TestListMagicTransitIPsecTunnels
--- PASS: TestListMagicTransitIPsecTunnels (0.00s)
=== RUN   TestGetMagicTransitIPsecTunnel
--- PASS: TestGetMagicTransitIPsecTunnel (0.00s)
=== RUN   TestCreateMagicTransitIPsecTunnels
--- PASS: TestCreateMagicTransitIPsecTunnels (0.00s)
=== RUN   TestUpdateMagicTransitIPsecTunnel
--- PASS: TestUpdateMagicTransitIPsecTunnel (0.00s)
=== RUN   TestDeleteMagicTransitIPsecTunnel
--- PASS: TestDeleteMagicTransitIPsecTunnel (0.00s)
PASS

Tested using a production account as well. The Create and Update endpoints occasionally run into a time out. I am checking with Magic team why that happens.

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]