Feature: Be able to provide structural information to improve accessiblity

**Is your feature request related to a problem? Please describe.**
Currently `react-pdf` does not expose any way to make the PDF more accessible while PDFKit the underlying library has some options to provide more structural information for visually impaired users who rely on screen readers/text-to-speech engines/vocalisation.

With PDFKit you are able to provide structural information as described in http://pdfkit.org/docs/accessibility.html.

**Describe the solution you'd like**

What I would like to see is a way to provide the `View` and `Text` primitives provided by react-pdf with the structural information. In react-native this can be achieved by properties like `accessibilityRole`.

**Describe alternatives you've considered**
Another solution could be providing more primitive elements for the different types of structural information, but if we expose those on the `View` and `Text` primitives a user could create those by wrapping the primitive components.

**Additional context**
@diegomura What is your opinion on this matter and the proposed solution?