Add Access application and policy support

[jacobbednarz]

This introduces two new resources for Cloudflare Access:

• cloudflare_access_application: An Access Application that covers the
  URL and a friendly name.
• cloudflare_access_policy: An Access Policy that enforces
  authorisation on aforementioned Application.

Initially I considered introducing a single resource
(cloudflare_access_application) however running over our existing
Access configuration showed that we would end up with quite a bit of
duplication if the policy was embedded in the resource. For example:

resource "cloudflare_access_application" "test"
  zone_id = "1d5fdc9e88c8a8c4518b068cd94331fe"
  name    = "my test"
  domain  = "test.envato-staging.com"

  policy {
    name = "allow test user"
    precedence = "1"
    decision  = "allow"

    include = {
      email = ["[email protected]"]
    }
  }
}

While it looks sensible here, it gets a bit messy with multiple policies
for a single application.

resource "cloudflare_access_application" "test"
  zone_id = "1d5fdc9e88c8a8c4518b068cd94331fe"
  name    = "my test"
  domain  = "test.envato-staging.com"

  policies = [
    {
      name = "prevent former employee access"
      precedence = "1"
      decision  = "deny"

      include = {
        email = ["[email protected]"]
      }
    },
    {
      name = "allow all users"
      precedence = "2"
      decision  = "allow"

      include = {
        email_domains = ["test.com"]
      }
    }
  ]
}

(Yes, this could be collapsed into a single policy block but bear with
me for the sake of the example)

Now, I'm not totally opposed to this approach however I was swayed away
from it since there are dedicated endpoints for the Application and
Policies and I thought this may cause some confusion if I coupled them
in the Terraform provider. If others think it should be embedded though,
I'm happy to update.

In it's current form, you'd need to create the two resources to
actually have a useful implementation. Example:

resource "cloudflare_access_application" "test_app" {
  zone_id = "1d5fdc9e88c8a8c4518b068cd94331fe"
  name    = "my test"
  domain  = "staging.example.com"
}

resource "cloudflare_access_policy" "test_policy" {
  application_id = "${cloudflare_access_application.test_app.id}"
  zone_id        = "${cloudflare_access_application.test_app.zone_id}"
  name           = "test policy"
  precedence     = "1"
  decision       = "allow"

  include = {
    email = ["[email protected]"]
  }
}

Screenshots

Access Application: https://shares.jacobbednarz.com/dfs98jdsf.png
Access Policy: https://shares.jacobbednarz.com/dfgsgdwf.png

Depends on cloudflare/cloudflare-go#244
Closes #100

cc @AustinCorridor

[patryk]

Yes, I agree with your design decision, this one looks better. Many thanks for contribution! 🍻