Add fvalidate to dogshell

Requirements for Contributing to this repository

Fill out the template below. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
The pull request must only fix one issue, or add one feature, at the time.
The pull request must update the test suite to demonstrate the changed functionality.
After you create the pull request, all status checks must be pass before a maintainer reviews your contribution. For more details, please see CONTRIBUTING.

What does this PR do?

Add fvalidate command to dogshell which validates a monitor using a file as input.

This PR addresses https://github.com/DataDog/datadogpy/issues/826

Alternate Designs

No other alternate designs considered.

Possible Drawbacks

N/A

Verification Process

Ran command locally against a monitor defined in a file.

monitor.json

{
    "name": "Sample monitor",
    "type": "query alert",
    "query": "avg(last_5m):avg:system.load.1{host:XYZ} by {host} > 5",
    "message": "Message @team",
    "tags": [],
    "options": {
        "thresholds": {
            "critical": 5,
            "warning": 1
        },
        "notify_audit": true,
        "include_tags": true,
        "new_group_delay": 60
    },
    "priority": 2,
    "restricted_roles": [
        "c3a0b9e2-ec22-11ee-be33-da7ad0900005"
    ]
}

➜  Desktop dog --api_host "datadoghq.eu" --pretty monitor fvalidate monitor.json
{}

bad-monitor.json

{
    "name": "Sample monitor",
    "type": "query alert",
    "query": "avg(last_5m):avg:system.load.1{host:XYZ} by {host} > 5",
    "message": "Message @team",
    "tags": [],
    "options": {
        "thresholds": {
            "warning": 1
        },
        "notify_audit": true,
        "include_tags": true,
        "new_group_delay": 60
    },
    "priority": 2,
    "restricted_roles": [
        "c3a0b9e2-ec22-11ee-be33-da7ad0900005"
    ]
}

➜  Desktop dog --api_host "datadoghq.eu" --pretty monitor fvalidate bad-monitor.json
{
  "errors": [
    "Alert status is required."
  ]
}

Additional Notes

N/A

Release Notes

Review checklist (to be filled by reviewers)

[ ] Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
[ ] PR title must be written as a CHANGELOG entry (see why)
[ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
[ ] PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
[ ] PR should not have do-not-merge/ label attached.
[ ] If Applicable, issue must have kind/ and severity/ labels attached at least.

DataDog / datadogpy

Add fvalidate to dogshell #827