Improve fleetctl apply validation and error handling

[xpkoala]

Fleet version: 4.50.0

Web browser and operating system: N/A

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💥  Actual behavior

When running an apply command via fleetctl (ex fleetctl apply -f <file>) the command will accept any file.

🧑‍💻  Steps to reproduce

1. With a fleetd binary created
2. run fleectl apply -f <fleet_binary.pkg>
3. An error will be displayed and then the full contents of the file will be streamed to stdout

🕯️ More info (optional)

• I have not been able to break the Fleet instance applying random files.
• I believe yaml files are the only file type that should be accepted here.

reed@reed fleet % cat bad.file 
no soup for you
reed@reed fleet % ./build/fleetctl apply -f bad.file
Error: failed to unmarshal spec item error unmarshaling JSON: json: cannot unmarshal string into Go value of type spec.Metadata: 
no soup for you

🛠️ To fix

• If a non yaml file is targeted for the apply command error out stating only yaml files can be applied.

[sharon-fdm]

Hey team! Please add your planning poker estimate with Zenhub @getvictor @jacobshandling @lucasmrod @mostlikelee @RachelElysia

[RachelElysia]

I would like this ticket :)

[fleet-release]

Improved validation, Fleetctl errors no more, Like clouds, quiet peace.