Add Attachments to the Products

[grigoer]

⚠️ Note on feature completeness ⚠️

Is your feature request related to a problem? Please describe
We have several documents that are connected to a product that we would like to have as attachment inside DefectDojo, since it would be easier to access them directly from DefectDojo as opposed to searching them elsewhere. For example User Manual Files, or some PowerPoint presentations that include important information. Right now attachments are only possible at Test level and main purpose there is for attaching Test Report.

Describe the solution you'd like
As a DefectDojo user I want to attach important Product Documents to the Product page on DefectDojo.

Describe alternatives you've considered
Alternatively it can be done via having separate "Documents" page where you can upload files and tag them with a product based tags. Tags should be automatically generated based on existing products and updated as soon as product is added/removed.

Additional context
For main solution the way attachment work for tests might be reused.