fix: correctly handle viper env var overrides for slice fields

[obs-gh-mattcotter]

Description

OB-40043: Correctly handle viper env var overrides for slice fields. We need this to specify host log overrides via env vars. This example works now but errored prior to this change:

$ env HOST_MONITORING::LOGS::INCLUDE='/test/file.txt,/file.log' ./observe-agent config
# ======== computed agent config
cloud_resource_detectors:
    - ec2
self_monitoring:
    enabled: true
host_monitoring:
    enabled: true
    logs:
        enabled: true
        include:
            - /test/file.txt
            - /file.log
    metrics:
        host:
            enabled: true

Checklist

• Created tests which fail without the change (if possible)
• Extended the README / documentation, if necessary

[obs-gh-alexlew]

what does the mapstructure flag do?

[obs-gh-mattcotter]

This tag is what viper checks for serialization. I found it through googling the exact problem we were hitting. We couldn't do direct viper (de)serialization without it because it would expect keys matching the variable name by default, which is why I did the yaml workaround that ultimately failed.