1255 Make fdc3.fileAttachment an independent type

[Yannick-Malins]

https://github.com/finos/FDC3/issues/1255

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[Yannick-Malins]

@kriswest

[kriswest]

@Yannick-Malins lemme know if you can make these two quick changes (I didn't add suggestions or I won't be able to review the change after you merge them) - alternatively let me know to re-raise this PR and have you review instead.

[kriswest]

Todo:

• [x] the filename and this $id need to match in fileAttachment.schema.json
• [x] define message entities with oneOf rather than anyOf in message.schema.json
• [x] add a changelog

[kriswest]

Thanks @Yannick-Malins. I'm not seeing the new type in the preview - but that could be because the PR is '199 commits behind finos/FDC3:main'. Could you merge changes from main then run npm run build. That should re-generate ContextTypes.ts so that goes in and the preview should generate correctly afterwards (those two are unrelated but we need to check both).

Otherwise looks all good to me. I'll get around to adding a PR template soon so the above steps are documented!

[Yannick-Malins]

should be good now!

[kriswest]

LGTM, thanks @Yannick-Malins

Docs page: https://deploy-preview-1261--fdc3.netlify.app/docs/next/context/ref/FileAttachment

Page that links to new docs page: https://deploy-preview-1261--fdc3.netlify.app/docs/next/context/ref/Message