fix: allow OTEL_CONFIG_OVERRIDES env var to replace agent config overrides

[obs-gh-mattcotter]

Description

OB-38640 allow OTEL_CONFIG_OVERRIDES env var to replace agent config overrides

This will allow the agent to be fully run with env variables only.

example:

$ OTEL_CONFIG_OVERRIDES=`cat tmp.yml` ./observe-agent config
# ======== computed agent config
host_monitoring:
    enabled: true
    logs:
        enabled: true
        include: []
    metrics:
        host:
            enabled: true
        process:
            enabled: false
observe_url: ""
otelconfigfile: ""
self_monitoring:
    enabled: true
token: ""


# ======== config file /var/folders/y8/v7cyk9611qbg059c0q02vk0w0000gn/T/observe-agent2529970453/otel-config-overrides-4110568018.yaml
processors:
    probabilistic_sampler:
        sampling_percentage: 50
service:
    pipelines:
        logs/forward:
            exporters:
                - debug
            processors:
                - probabilistic_sampler
            receivers:
                - otlp

Checklist

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

[obs-gh-alexlew]

does this handle nested configs correctly? we have a different delimiter :: for nested keys

[obs-gh-alexlew]

https://opentelemetry.io/docs/collector/configuration/#override-settings similar to how otel does it (this is because there can be periods within a key name in the config so that overloads '.' as the delimiter

[obs-gh-mattcotter]

The file in this case if full YAML, this was requested here: https://observeinc.slack.com/archives/C06MM0MSHPA/p1730998713772329?thread_ts=1730939763.831879&cid=C06MM0MSHPA

$ cat tmp.yml 
processors:
  probabilistic_sampler:
    sampling_percentage: 50.0

service:
  pipelines:
    logs/forward:
      receivers: [otlp]
      processors: [probabilistic_sampler]
      exporters: [debug]

[obs-gh-alexlew]

so what's the actual value we provide in this env var? what does cat tmp.yml look like when expanded?