Allow Update of Only Custom Templates

[kchason]

Please describe your feature request:

Add a new flag that allows custom templates to be updated without updating the default templates from the nuclei-templates project on GitHub.

This would be reflected with a new CLI argument:

-uct, -update-custom-templates    update custom templates to the latest available version(s)

Intended Behavior:

• Default or -ut: both the nuclei-templates and custom template providers are checked for updates and downloaded as appropriate (current behavior).
• -duc: no templates are downloaded and no checks are performed (current behavior).
• -duc -ut: no templates are downloaded and no checks are performed (current behavior).
• -duc -uct: only the custom template providers (if available) are checked and downloaded (proposed behavior).

This is addressed in: #3706

Describe the use case of this feature:

1. Allows nuclei to update the custom templates from local resources while not on the open internet.
2. Reduces the number of calls and data traffic required during updates when the user only wants to update custom templates.