Add maintenance mode CLI commnds

Problem/Motivation

Part of #3582246: [meta] CLI in Core community initiative. Referenced from the CLI in Core Initiative spreadsheet.

Port 2 of 3 commands from Drush. I decided to combine maint:get and maint:status. maint:status now does both duties, and the command help explains how to get the status code using `|| true` if you need it for scripting.

Remaining tasks

None

User interface changes

None

Introduced terminology

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API changes

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Data model changes

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Release notes snippet

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