Define and implement event kind to store Trustroots data

[shuesken]

We will need to store basic information about Trustroots users in Nostr so that it can be used by the app and ultimately by other services. In the long-run, we will want no information to be exclusively storeable on Trustroots servers. In the short-term, we should at least put Circles into Nostr so we can use it for pre-filtering the map.

We're also having to deal with #31 so the same event should probably also contain the trustroots username.

In the mid-term, the app (or the website) will require an extra opt-in dialogue for whether we can publicize personal data in this way.

This issue is complete when a new event kind (or event kinds) are defined in nr-common and used by the app and server to store some trustroots profile data.