Add requested_path to readmeHtml field resolver

What does this MR do and why?

When trying to load an image from a relative path in a catalog version's README, the links appear to be broken.

This is happening because somewhere in the markup/down processing code we are losing the requested_path for the image, which is needed in order for it to be rendered raw and displayed properly.

This MR is the smallest change to fix the images - it simply adds a postprocess option to the markdown method in MarkupHelper that merges the missing requested_path value with the postprocessing_context before sending it to the Markup::RenderingService.

Further investigation along with a refactoring will be done in an immediate follow up: #460462 (closed). In this refactoring we will also be using a proper markdown_field in GraphQL and cache the markdown for performance with a database migration.

Resolves: #455516 (closed)

Screenshots or screen recordings

Before

Note that this only happens with relative links, the second image is not relative:

After

Image shows up whatever the path is:

How to set up and validate locally

In a project marked as a catalog resource, include an image with a relative path in the README and make a release.

For an example, you can see gitlab.com/components/ruby. The README in the project looks totally fine, but if you go to the latest release's README: https://gitlab.com/explore/catalog/components/ruby, you will be able to see the broken links.