Ruleset_managed_challenge_1436

[vences]

For #1436

Following change in go #1442 I added acceptance test and also updated the documentation for the action managed_challenge in rulesets

[vences]

➜  terraform-provider-cloudflare git:(ruleset_managed_challenge_1436) ✗ TF_ACC=1 go test $(go list ./...) -v -run TestAccCloudflareRuleset_WAFManagedRulesetWithActionManagedChallenge
?       github.com/cloudflare/terraform-provider-cloudflare     [no test files]
=== RUN   TestAccCloudflareRuleset_WAFManagedRulesetWithActionManagedChallenge
=== PAUSE TestAccCloudflareRuleset_WAFManagedRulesetWithActionManagedChallenge
=== CONT  TestAccCloudflareRuleset_WAFManagedRulesetWithActionManagedChallenge
--- PASS: TestAccCloudflareRuleset_WAFManagedRulesetWithActionManagedChallenge (19.87s)
PASS
ok      github.com/cloudflare/terraform-provider-cloudflare/cloudflare  20.167s
?       github.com/cloudflare/terraform-provider-cloudflare/tools/cmd/changelog-check   [no test files]
?       github.com/cloudflare/terraform-provider-cloudflare/version     [no test files]

[github-actions]

Oops! It looks like no changelog entry is attached to this PR. Please include a release note as described in https://github.com/cloudflare/terraform-provider-cloudflare/blob/master/docs/changelog-process.md.

Example:

```release-note:TYPE
Release note
```

If you do not require a release note to be included, please add the workflow/skip-changelog-entry label.

[jacobbednarz]

thank you @vences, this is perfect!