cli: add -json and -t flags to operator autopilot get-config

[singhvishalkr]

I noticed that nomad operator autopilot get-config is missing the -json and -t flags that other inspection commands have. Scripting and automation tools benefit from machine-readable output instead of parsing the default key=value lines.

This PR adds both flags following the same pattern used by nomad server members and other commands:

• -json outputs the full AutopilotConfiguration struct as JSON
• -t accepts a Go template for custom formatting

Two tests are included to verify the JSON unmarshals correctly and that templates work as expected.

Addresses #15894 (partial)

[hashicorp-cla-app]

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[hashicorp-cla-app]

Thank you for your submission! We require that all contributors sign our Contributor License Agreement ("CLA") before we can accept the contribution. Read and sign the agreement

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[singhvishalkr]

I have read and signed the CLA.

[tgross]

@singhvishalkr to get the CLA bot to approve, you need to have signed with the same email address as the commit. (See https://patch-diff.githubusercontent.com/raw/hashicorp/nomad/pull/27991.patch if you're not sure which one that is)

[singhvishalkr]

@tgross Every commit on this branch already lists vishal.kr.singh2021@gmail.com as author email. If the CLA badge still shows pending, I will trigger the HashiCorp CLA recheck after the next sync; the address matches what I used when signing.