Add missing otel signal viewer plugin for the otel Agent

[ralphm]

As the default configuration explicitly disables "new" plugins, all plugins that are not explicitly enabled will be disabled. Since the set of enabled plugins for use within k8s has been created a while ago, new plugins such as the otel-signal-viewer plugin are not mentioned, even though we do expect them to be there.

[CLAassistant]

All committers have signed the CLA.

[ralphm]

@M4itee We may want to add some other plugins as well, like network-viewer. This problem will continue to pop up when we add new plugins.

We currently have enable running new plugin = no, which means 'disable all by default', so all the explicit = no entries we have now are basically no-ops. Possibly, we could have that setting as 'yes' and then only disables the plugins we do not want?

[M4itee]

we can, the goal was for this endpoint to ONLY host logs (no overhead)

[ralphm]

Per out-of-band discussion, this change should be sufficient for the time being. Please review.