Add data source for WAF Rules

[XaF]

Following up on #506 and #508, and as explained in comment https://github.com/terraform-providers/terraform-provider-cloudflare/pull/508#issuecomment-540235472, we need to be able to bulk enable/disable rules too. As the resource already existed I did not pay attention that we also needed to create the data source previously... But realized that missing part while updating our terraform to the last 2.0.1 release!

This thus adds the new data source and related documentation and acceptance tests.

=== RUN   TestAccCloudflareWAFRules_NoFilter
=== PAUSE TestAccCloudflareWAFRules_NoFilter
=== RUN   TestAccCloudflareWAFRules_MatchDescription
=== PAUSE TestAccCloudflareWAFRules_MatchDescription
=== RUN   TestAccCloudflareWAFRules_MatchMode
=== PAUSE TestAccCloudflareWAFRules_MatchMode
=== CONT  TestAccCloudflareWAFRules_NoFilter
=== CONT  TestAccCloudflareWAFRules_MatchMode
=== CONT  TestAccCloudflareWAFRules_MatchDescription
--- PASS: TestAccCloudflareWAFRules_MatchMode (6.40s)
--- PASS: TestAccCloudflareWAFRules_NoFilter (6.66s)
--- PASS: TestAccCloudflareWAFRules_MatchDescription (6.69s)
PASS
ok  	github.com/terraform-providers/terraform-provider-cloudflare/cloudflare	6.720s