Add a schema for attachments

[leonard84]

Motivation

I think the use-case to have arbitrary attachments should be universal enough to merit inclusion in OTR directly, instead of every implementer creating a custom solution for this problem.

Implementation suggestion:

We could either have a generic attachment element that has html-attributes like src=URL, type=[image,video,file],
or we could have dedicated elements image[Attachment], video[Attachment], ...

The CLI tool should probably support moving local attachments to a new folder structure.

[marcphilipp]

I'm in favor of a generic attachment which can either point to an external location via a URL or (for small files) its content can be included directly inside the report in base64-encoded form.

[marcphilipp]

WIP PR: junit-team/junit5#3336