Feature: Add underMaintenance check API

[minxylynx]

Summary

Add an API to respond with an underMaintenance flag

Proposal

• Add ENV for isUnderMaintenance
• Add API to return the isUnderMaintenance flag
• Use isUnderMaintenance to shut off auto ingest, calls at height, etc

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned