sd-dlopen: make macros to generate .notes.dlopen sections public API (#41047)

If this new scheme of adding dependencies is supposed to be used more
widely we need to start making it easy to add them. So add a new
self-contained header that projects can simply include without the need
to link against libsystemd itself. This will allow them to generate
`.notes.dlopen` sections:

```
> readelf -p .note.dlopen ./l2md

String dump of section '.note.dlopen':
  [     a]  |@FDO
  [    10]  [{"feature":"manifest-json","description":"Manifest-based change detection via gzip and JSON parsing","priority":"suggested","soname":["libz.so.1","libsystemd.so.0"]}]
  [    c2]  |@FDO
  [    c8]  [{"feature":"manifest-http","description":"HTTP transport for lore.kernel.org manifest fetch","priority":"suggested","soname":["libcurl.so.4"]}]
```