feat: add support for URL monitors [sc-24803]

[sorccu]

This PR adds support for a new resource type: checkly_url_monitor - URL monitor.

resource "checkly_url_monitor" "test" {
  name      = "test"
  activated = true
  frequency = 30
  locations = ["us-east-1", "eu-central-1"]

  request {
    url              = "https://welcome.checklyhq.com"
    follow_redirects = false

    assertion {
      source     = "STATUS_CODE"
      comparison = "EQUALS"
      target     = "200"
    }
  }
}

I've taken care to add a bit more documentation and validation this time, since the pending TF provider overhaul is likely still months away.

Additionally, checkly_heartbeat has been renamed to checkly_heartbeat_monitor, and checkly_tcp_check to checkly_tcp_monitor. The old resource types are still available, and have not been deprecated. If we deprecate them, users will start seeing warnings every time they deploy their resources. In the Plugin SDK however there is no convenient way to move a resource to a new type, so the steps to rename a resource are a bit convoluted:

1. Figure out the ID of the affected check
2. Rename the resource (name, type) in the .tf file
3. Use the import command or preferably the import block to import the existing resource into the new resource type
4. Use terraform state rm to remove the old resource from TF state

With the framework, a single "moved" block would do the job (well, we'd have to have extra code for it in the provider though).

So, for now, their descriptions will say that they've been renamed, but there's no outright deprecation.

Of course, if you don't care about keeping historical data, you can just recreate the resources by renaming them.

Affected Components

• Resources
• Test
• Docs
• Tooling
• Other

Pre-Requisites

• Terraform code is formatted with terraform fmt
• Go code is formatted with go fmt
• plan & apply command of demo/main.tf file do not produce diffs

Notes for the Reviewer

Resolves #[issue-number]

New Dependency Submission

[github-actions]

Terraform Format and Style 🖌success

Format Output

[sujaya-sys]

Suggested change

	- `target` (String) The target value. Typically `200` when the source is `STATUS_CODE`.
	- `target` (String) The target value. Use a 2xx value to assert a successful `STATUS_CODE` response, e.g. `200`.

Might be worth highlighting that the assertion doesn't work for negative status codes (this is something we need to fix / add validation for at some point)