Add support for file comments in spdx

[alpianon]

For an open source project I'm working on (Oniro), I added initial support for comments on single files, allowed both by dep5 and spdx specs:

• added 'comment' field to SpdxInfo
• added conversion of dep5 Files:Comment fields into SPDX FileComment fields
• TODO: it would be useful to be able also to add (human-readable) comments with reuse addheader

A typical (but not the only one) use case is to handle cases in which different portions of a file are subject to different licenses (I'm finding a lot of such cases with patch files in yocto layers). The official FAQ suggest to use comments to that purpose https://reuse.software/faq/#partial-license

Specific tests are still missing. If there is consensus on this PR, I will add them (as proposed to @linozen )

[mxmehl]

Sorry for the late review. Please note that for marking portions of a file being under a different license, SPDX snippets are the way to go. They have been introduced in SPDX 2.3. We will also cover this in the next spec release.

Here's the current status: https://github.com/fsfe/reuse-docs/blob/112d289667f2c4d315613a59ac467f9a6a3f45ef/spec.md?plain=1#L165

The FAQ item you referred to will also be updated: fsfe/reuse-docs#124

Apart from that, covering SPDX comments seems to be OK. But please note that there has been some work done on the SPDX output in a different PR which probably causes the conflict. I'm also not sure whether it conflicts with the big work going on in #654.

[linozen]

Since #654 was merged, there now is a dataclass called ReuseInfo to which SPDX comments need to be added. Is this still relevant, @alpianon? If yes, this PR would need to be rebased on main and specific tests should be added.