[JENKINS-70684] Deprecate standalone use of hudson.Main

[daniel-beck]

See JENKINS-70684.

Thoughts about this as a first step?

Testing done

None yet, so far asking about the general direction.

Proposed changelog entries

• Deprecate use of jenkins-core.jar for External Monitor Job Type Plugin.

Proposed upgrade guidelines

N/A

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Desired reviewers

@mention

Before the changes are marked as ready-for-merge:

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[jglick]

Seems right, and if there is functionality that cannot be handled by a CLI command for whatever reason as #8825 (comment) implies (AFAICT this would only be including the command exit status automatically, as you can already pipe command output into the CLI) then it would be better to create an entry point main class in the plugin instead.