POC: synthetics_global_variable: write only value

[bernata]

For the datadog_synthetics_global_variable resource, create value_wo and value_wo_version attributes. value_wo is a wrote only attribute that behaves like the value attribute [except write only]. This will support writing ephemeral resources to a global variable. The reason this is important is ephemeral resources and write only attributes do not get written to the terraform state file. These secrets can be sensitive [like credentials used by a synthetic] and are frequently forbidden by security policy to live in terraform state backends.

[bernata]

For the datadog_synthetics_global_variable resource, create value_wo and value_wo_version attributes. value_wo is a wrote only attribute that behaves like the value attribute [except write only]. This will support writing ephemeral resources to a global variable. The reason this is important is ephemeral resources and write only attributes do not get written to the terraform state file. These secrets can be sensitive [like credentials used by a synthetic] and are frequently forbidden by security policy to live in terraform state backends.

@LiuVII I noticed you created a similar POC. I was wondering what your thoughts were on continuing this one and if datadog would accept it?