… feedback (elastic#50797)

## Summary

Updated REST API from feedback from the UI/UX
  * Changes the `id` to be `rule_id` on PUT/POST and makes it optional for a POST (create).
  * On data return sets both `id` and `rule_id` is returned. If `rule_id` is not set, a uuid.v4() will b assigned to the rule_id and the value will be returned.
  * Transforms output of all endpoints to be 1-1 to the input.
  * Fixes delete to return the deleted rule
  * Changes the URL to be `/api/detection_engine/rules`
  * Changes the POST behavior to fail with a `409 (conflict)` if the rule already exists (For creates)
  * Changes the POST behavior where sending in a `rule_id` is now optional. If none are sent in it does not create a `rule_id` and instead returns `null` for the `rule_id` and the autogenerated one.
  * Changes the PUT behavior to fail with a `404 (not found)` if the rule does not already exist (For updates)
  * Deletes the actions code and just uses an empty array since we don't have actions yet
  * Makes all error conditions consistent and does not expose the underlying error codes. Only exception to the rule is if an error condition returns non `404` or something unexpected. In which case it will show that error upstream.


Example post output:

```ts
$ ./post_signal.sh
{
  "created_by": "elastic",
  "description": "Detecting root and admin users",
  "enabled": true,
  "false_positives": [],
  "from": "now-6m",
  "id": "8277a0e8-474c-4507-9c11-5f197b5fe2d5",
  "index": [
    "auditbeat-*",
    "filebeat-*",
    "packetbeat-*",
    "winlogbeat-*"
  ],
  "interval": "5m",
  "rule_id": "rule-1",
  "language": "kuery",
  "max_signals": 100,
  "name": "Detect Root/Admin Users",
  "query": "user.name: root or user.name: admin",
  "references": [
    "http://www.example.com",
    "https://ww.example.com"
  ],
  "severity": "high",
  "updated_by": "elastic",
  "tags": [],
  "to": "now",
  "type": "query"
}
```

Example delete and get URL's now (see scripts for more details):

```ts
${KIBANA_URL}/api/detection_engine/rules?rule_id="rule-1"
${KIBANA_URL}/api/detection_engine/rules?id="04128c15-0d1b-4716-a4c5-46997ac7f3bd"
```

### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

~~- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)~~

~~- [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)~~

~~- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~~

- [x] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios

~~- [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~~

### For maintainers

~~- [ ] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

- [x] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)