[CDTOOL-1098] Add support for ngwaf workspaces

[philippschulte]

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[philippschulte]

@kpfleming thanks for completing the PR! Both test pass:

TF_ACC=1 go  test $(go  list ./...) -v -run=TestAccFastlyNGWAFWorkspace_validate -parallel=4 -timeout 360m -ldflags="-X=github.com/fastly/terraform-provider-fastly/version.ProviderVersion=acc"
?   	github.com/fastly/terraform-provider-fastly	[no test files]
=== RUN   TestAccFastlyNGWAFWorkspace_validate
=== PAUSE TestAccFastlyNGWAFWorkspace_validate
=== CONT  TestAccFastlyNGWAFWorkspace_validate
--- PASS: TestAccFastlyNGWAFWorkspace_validate (11.85s)
PASS

TF_ACC=1 go  test $(go  list ./...) -v -run=TestAccFastlyDataSourceNGWAFWorkspaces_Config -parallel=4 -timeout 360m -ldflags="-X=github.com/fastly/terraform-provider-fastly/version.ProviderVersion=acc"
?   	github.com/fastly/terraform-provider-fastly	[no test files]
=== RUN   TestAccFastlyDataSourceNGWAFWorkspaces_Config
=== PAUSE TestAccFastlyDataSourceNGWAFWorkspaces_Config
=== CONT  TestAccFastlyDataSourceNGWAFWorkspaces_Config
--- PASS: TestAccFastlyDataSourceNGWAFWorkspaces_Config (12.68s)
PASS