Allow ApplicationSets in any namespace without setting allowed-scm-providers

[tculp]

Summary

Currently, if you try to enable the "ApplicationSets in any namespace" feature, you must set the allowed-scm-providers config, otherwise you get an error. However, not setting that field should work and perhaps just print a warning that no scm providers are enabled.

I am using ArgoCD v2.8.1.

Motivation

I don't want any scm providers to be allowed, so I have to set the allowed-scm-providers field to a dummy URL. An empty string did not work.

Proposal

Default an unset allowed-scm-providers as assuming that no SCM providers are allowed.

[crenshaw-dev]

Instead of interpreting and empty --allowed-scm-providers to mean "allow none" (a breaking change), I recommend we introduce a new --disable-scm-generator flag to improve the UX.

[tculp]

That would work too