fix(fastly_service_vcl): validate snippet names

[Integralist]

Problem

A user can define multiple snippets with the same name attribute.
This causes the Fastly API to treat the data using 'last entry wins' behaviour.
Resulting in invalid plan diffs and state changes.

Solution

The Fastly Terraform provider should validate that snippet names are unique.

Notes

I checked the schema.Schema type to see if there were any useful validation fields, and although there is ValidateFunc it only validates that specific instance of a block type and so it isn't useful for comparing across separate blocks within a resource.

Because of this I had to resort to adding extra validation via the existing fastly_service_vcl resource's CustomizeDiff.

Fixes #672