[JENKINS-72371] rework node monitor configuration

[mawinter69]

This PR reworks the way node monitors are configured. It ensures that also monitors which are set to ignored, can be configured.

Previously, there was a checkbox before each monitor, where the behaviour was not totally clear. It meant that the monitor is ignored but still collecting data and not disabled as one might think. But when the monitor was ignored any configuration was lost and default values were used. Due to this behaviour the Versions Node Monitor plugin implemented its own disconnect checkbox.

Before:

After:

See JENKINS-72371.

Testing done

Before:

1. Go to computer/configure
2. Set Value for Free Temp Space to 2 GiB and 3 GiB warning, apply and reload page
3. Uncheck Free Space Space (the config fields are no longer visible) apply and reload page
4. Check the file nodeMonitors.xml in JENKINS_HOME, there are the default values of 1GiB and 2 GiB warning.
5. Check Free Space Space in the UI, the config fields reappear and contain the defaults

After:

1. Go to computer/configure
2. Set Value for Free Temp Space to 2 GiB and 3 GiB warning, apply and reload page
3. Check Don't Mark agents temporarily offline for Free Space Space apply and reload
4. Check the file nodeMonitors.xml in JENKINS_HOME, there values of 2GiB and 3GiB warning are still there
5. The UI shows the configured values and not the defaults

Proposed changelog entries

• JENKINS-72371, rework node monitor configuration

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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[Kevin-CB]

Looks fine security wise.

[NotMyFault]

looks neat

[timja]

(Removed ready for merge as I see it was flagged as needing ATH)

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!