Provide a non @addtaghelper way to add TagHelper descriptors

[NTaylorMullen]

Right now you can do this by replacing the entire TagHelperDescriptorResolver which is obviously pretty heavy handed so we should put in an extensibility point to support this.

[Eilon]

@DamianEdwards @NTaylorMullen @rynowak is this a real thing? Who would use this?

[NTaylorMullen]

Orchard would use this for custom TagHelper types (shapes); however, when this issue was initially written it was to address problems like app-insights/glimpse needing to inject script tags in the body without using an @addTagHelper.

I'd say it's still valuable even for us since we'd use it for VIewComponentTagHElpers (it's hacked together to make that work today).

[Eilon]

We have a different solution for Application Insights and Glimpse, which is to have a default head/body tag helper that they can inject content into: aspnet/Mvc#5728

Is it a big chunk of design work to come up with something cleaner for VCTH's?

[NTaylorMullen]

Is it a big chunk of design work to come up with something cleaner for VCTH's?

Ehh, not too much but not free. I can see this issue as open or closed; we've lived this long without a first class API. Can live a while longer 😄