Add TimestampDatetimeWidget settings for readonly

@mably, thanks for you inquiry. Our specific use case is that our editors and publishers would like to be able to see the publication date data, but they should not be able to alter that data. For example, if a content author needed to report back the publication date of a piece of content for compliance reasons, it would be helpful to be able to see that date using the field widget, but they should not be able to alter that publication date. I see a permission that allows the widget to be displayed, but there is no granular controls for controlling editing. Hope that clarifies the use case.

@mably, a few thoughts on the widget and permissions implementation. This widget seems to work differently compared to other field widget implementations I've encountered. Other field widgets don't rely on permissions for widget visibility on the form display. Most widget leverage the form display itself, i.e. if you need to hide the widget you would disable it in manage form display. Additionally, I like the flexibility of having the editing of the widget configurable on a per content type basis, which gives more flexibility on a content type basis. Interested on your thoughts or background on the existing approach.

@mably, oh you are correct the permissions are on a per content type basis. This would allow for flexibility given different roles. I can look at updating the MR to user permissions for editing rather than widget settings.

@mably, I've made that update per our conversation. Please feel free to review at your convenience.

@mably, I've added a lightweight functional test to tests the permissions for the module.

@mably, I've fixed those phpcs warnings and errors. Thanks for catching that.

Thanks @mably, great working with you on this!