Add filter by cluster uuid in subnet datasource

[shreevari]

Fixes #308

[yannickstruyf3]

Can we create a generic "filter" param so it is easy to extend it with additional parameters down the road? For example filter subnets based on VLAN ID?
This same filter logic can be applied to other data sources as well. Similar example: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/ami

[shreevari]

AWS provider uses a common ec2 filter struct and transparently passes them to the SDK, thus being handled by SDK/API.

An equivalent of this in Nutanix V3 APIs is the filter param in the metadata, which we already expose for list type data sources.
Do you suggest that I add a filter param for single data sources like nutanix_subnet, nutanix_cluster, etc and pass the filter through to the API call?

[shreevari]

Here's an example of filtering for list type data sources that I tried out for filtering by vlan_id

data "nutanix_clusters" "clusters" {}

output "sbnet" {
  value = data.nutanix_subnets.test
}

data "nutanix_subnets" "test" {
  metadata {
    filter = "vlan_id==154"
  }
}

[siddharth-nutanix]

@shreevari Can you also add an example and update the datasource documentation for this change?

[siddharth-nutanix]

/ok-to-test

Acceptance test run status: success
Line code coverage is 60.2

[yannickstruyf3]

Can we also add a test case for filters in data_source_nutanix_subnet_test.go?