feat(datasource/sites): support sites list

[Integralist]

Fixes part of @sjparkinson issue reported here #111

terraform {
  required_providers {
    sigsci = {
      source  = "signalsciences/sigsci"
      version = "1.2.20"
    }
  }
}

resource "sigsci_site" "example1" {
  short_name   = "terraform_site1"
  display_name = "terraform test example site1"
}

resource "sigsci_site" "example2" {
  short_name   = "terraform_site2"
  display_name = "terraform test example site2"
}

resource "sigsci_site" "example3" {
  short_name   = "terraform_site3"
  display_name = "terraform test example site3"
}

data "sigsci_sites" "example" {
  depends_on = [sigsci_site.example1, sigsci_site.example2, sigsci_site.example3]
  filter     = "terraform test example site1" # checks against `name` and `display_name`
}

[Integralist]

Comments for the reviewer...

[Integralist]

This enables me to locally test the provider.

[Integralist]

NOTE: The API has a blocklist field but it's not implemented in the go-sigsci API client.

[Integralist]

Previously I was dynamically generating a dynamic ID but opted for a static ID as I don't imagine there will be multiple instances of the sigsci_sites data source. If we do think that is a possibility then we should make this dynamically generated (possibly using something like https://github.com/google/uuid to generate the ID).

[Integralist]

The implementation for the filter attribute is 'match only one', e.g. the first site that matches is returned.

If we want the filter to return multiple potential sites, then the implementation should be updated.

[Integralist]

My editor made a bunch of automated refactors.

[shawnps]

LGTM, thank you @Integralist!