Support validating monitors using files

[kkyr]

Note:
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Is your feature request related to a problem? Please describe.
dogshell currently only allows validating monitors by explicitly passing parameters, which can be cumbersome when attempting to validate monitors defined in files.

Describe the solution you'd like
I suggest we add a new command fvalidate which allows validation of a monitor by passing a file, similar in fashion to fpost and fupdate.

Describe alternatives you've considered
No other alternatives considered

Additional context
Add any other context or screenshots about the feature request here.

[github-actions]

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[kkyr]

Issue is still reproducible.