Add Accept Header to Endpoint indicating acceptable content types

[dshanske]

https://webmention.net/draft/#request-verification

This would help in expanding per media type rules

[github-actions]

This issue is stale because it has been open 365 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[github-actions]

This issue is stale because it has been open 365 days with no activity. Remove stale label or comment or this will be closed in 5 days.