Add lint warning for unused extensions

[garysassano]

Currently, when an extension is defined but never used, there is no clear warning. To improve usability, the warning message could follow the style of ESLint's unused variable warnings.

Example Configuration

receivers:
  otlp:
    protocols:
      grpc:
      http:

processors:
  batch:

exporters:
  otlp:
    endpoint: otelcol:4317

extensions:
  health_check:
  pprof:
  zpages:

service:
  extensions: [pprof, zpages]
  pipelines:
    traces:
      receivers: [otlp]
      processors: [batch]
      exporters: [otlp]
    metrics:
      receivers: [otlp]
      processors: [batch]
      exporters: [otlp]
    logs:
      receivers: [otlp]
      processors: [batch]
      exporters: [otlp]

Expected Output

warning: 'health_check' is defined but never used. (unused-extension)

This would make it easier to identify unused extensions. I had an issue where I had lots of extensions and I forgot to include one in the service section. I spent half an hour debugging an issue that could have been resolved immediately.