feat: automatically search for in-cluster specs when none are supplied

[xavpaice]

Describe the rationale for the suggested feature.

When folks run support-bundle --load-cluster-specs, it seems redundant to need to add the CLI switch given that they have not supplied an argument (or - for stdin) to specify where the spec should be.

Describe the feature

Set a default so that the URI/spec location is optional. If not supplied, automatically set --load-cluster-specs to true.

Describe alternatives you've considered

Describe alternative solutions here. Include any workarounds you've considered.

Additional context

Add additional context about the feature request. If the change is substantial, consider attaching files to the issue outlining architectural changes, data flows, file formats etc., anything that helps describe the requested change.