now Pleroma is special here, it supports following like this in bog-standard ActivityPub!
ie just posting a small json payload to the logged-in user's outbox endpoint
hopefully more ActivityPub servers support this soon, as it should be very similar to what they already do for S2S
misskey and micro.blog from @manton both support client registration for clients like this by simply providing a website url
it's called "indieauth" (displayed in the login info row, otherwise falls back to mastodon-style app-creation-based registration)
there is a newer emerging standard for dynamic client registration, but no one supports it yet - I'll add it when one does!
micro.blog doesn't support C2S follows via the outbox yet, but we can fallback to the micro.blog api here