feat: add 'config validate' command to diagnose config issues without other checks

[obs-gh-mattcotter]

Description

Add 'config validate' command to diagnose config issues without other checks. This can be used by our helm chart to check all of the config maps.

Examples:

$ ./observe-agent config validate 
❌ invalid config
Error: service::pipelines::metrics/agent-filestats: references exporter "otlphttp/observemetrics2" which is not configured

# after fixing my config
$ ./observe-agent config validate 
✅ configuration is valid

This also adds a hidden config-mode flag so you can (e.g.) validate the bundled docker config from a mac build:

$ ./observe-agent config --render-otel | grep eventlog

$ ./observe-agent --config-mode windows config --render-otel | grep eventlog
    windowseventlog/host_monitoring-application:
    windowseventlog/host_monitoring-security:
    windowseventlog/host_monitoring-system:
                - windowseventlog/host_monitoring-application
                - windowseventlog/host_monitoring-security
                - windowseventlog/host_monitoring-system

[orca-security-us]

Orca Security Scan Summary

Status	Check	Issues by priority
Passed	Infrastructure as Code	0   0   0   0	View in Orca
Passed	SAST	0   0   0   0	View in Orca
Passed	Secrets	0   0   0   0	View in Orca
Passed	Vulnerabilities	0   0   0   0	View in Orca

[obs-gh-alexlew]

is this a built in command? that's cool

[obs-gh-mattcotter]

It is! The vanilla otelcol has this validate command. We have this exact same call in diagnose as well, but I figured having a simple config check was valuable in addition to the full suite (especially for the helm chart case).