Implement support for Spanner Granular Instances

[davidcueva]

Context:
https://cloud.google.com/blog/products/databases/get-more-out-of-spanner-with-granular-instance-sizing

[davidcueva]

• Add new parameters to the Poller
• Add new parameters to the Scaler
• Add support in scaling algorithms
• Testing plan
• Update docs
• Update Terraform Examples