azurerm_data_factory_linked_service_sftp: support for hostkey related properties

[favoretti]

Add skip_host_key_validateion and host_key_fingerprint arguments.

$ TF_ACC=1 go test -v ./azurerm/internal/services/datafactory -timeout=1000m  -run='TestAccDataFactoryLinkedServiceSFTP'
2021/05/22 13:03:13 [DEBUG] not using binary driver name, it's no longer needed
2021/05/22 13:03:13 [DEBUG] not using binary driver name, it's no longer needed
=== RUN   TestAccDataFactoryLinkedServiceSFTP_basic
=== PAUSE TestAccDataFactoryLinkedServiceSFTP_basic
=== RUN   TestAccDataFactoryLinkedServiceSFTP_update
=== PAUSE TestAccDataFactoryLinkedServiceSFTP_update
=== CONT  TestAccDataFactoryLinkedServiceSFTP_basic
=== CONT  TestAccDataFactoryLinkedServiceSFTP_update
--- PASS: TestAccDataFactoryLinkedServiceSFTP_basic (117.42s)
--- PASS: TestAccDataFactoryLinkedServiceSFTP_update (162.56s)
PASS
ok  	github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/datafactory	164.318s

Fixes #11797

[favoretti]

[magodo]

@favoretti Thanks for this PR!
I've taken a look through and left some comments inline. Besides which, does it make sense to add test cases for the new properties?

[magodo]

@favoretti LGTM 👍 Thank you!

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.