feat(resource): add azurerm_api_management_custom_domain

[haarchri]

Description of your changes

add azurerm_api_management_custom_domain

Running uptest for this resource is not feasible because we require the principal_id from the system assigned identity of APIM to add an AccessPolicy for Azure KeyVault. We can't use user assigned identities due to an existing issue in the tf provider, as detailed in this link: hashicorp/terraform-provider-azurerm#18323.

Fixes #626

I have:

• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

local via uptest with manual interaction

kubectl get managed
NAME                                                               READY   SYNCED   EXTERNAL-NAME                       AGE
resourcegroup.azure.upbound.io/custom-domain-example-op-dkrumhk7   True    True     custom-domain-example-op-dkrumhk7   6h45m

NAME                                                                          READY   SYNCED   EXTERNAL-NAME                       AGE
management.apimanagement.azure.upbound.io/custom-domain-example-op-0chrwp0h   True    True     custom-domain-example-op-0chrwp0h   6h45m

NAME                                                                READY   SYNCED   EXTERNAL-NAME   AGE
customdomain.apimanagement.azure.upbound.io/custom-domain-example   True    True     default         116m

NAME                                                                      READY   SYNCED   EXTERNAL-NAME                                            AGE
certificate.keyvault.azure.upbound.io/custom-domain-example-op-g3wioohj   True    True     custom-domain-example/4e87d90834d64e5fb63cc8217e73de2b   3h12m

NAME                                                   READY   SYNCED   EXTERNAL-NAME          AGE
vault.keyvault.azure.upbound.io/cdexampleop-9clnwhyy   True    True     cdexampleop-9clnwhyy   3h20m

NAME                                                                READY   SYNCED   EXTERNAL-NAME                          AGE
accesspolicy.keyvault.azure.upbound.io/custom-domain-example        True    True     32a93987-5591-4177-9edc-459c03cbc4be   3h12m
accesspolicy.keyvault.azure.upbound.io/custom-domain-example-apim   True    True     030f645f-5f58-47ee-b061-e0317830772d   128m

[turkenf]

Thanks for this PR @haarchri, update, import, and delete scenarios tested successfully. Just a minor comment ⬇️
Fix for YAML file name under the examples/apimanagement: apicustomdomain.yaml => customdomain.yaml