Implements core NIP-26 delegated event functionality. Events can
include a `delegation` tag that provides a signature and restrictions
on which events can be delegated.
Notable points on the implementation so far:
* Schema has been upgraded to include an index and new column.
* Basic rune parsing/evaluation to implement the example event in the
NIP, but no more.
* No special logic for deletion.
* No migration logic for determining delegated authors for
already-stored events.
Schema upgrades were buggy from 4->5 (the v5 would be skipped). This
change also refactors the logic slightly so that future additions can
be clearer (no need to have if and else-if combinations).
Only lowercase and even-length tag values are stored as binary BLOBs.
Previously there was an error which search results from being returned
if the tag value was mixed-case and could be interpreted as hex.
A new database migration has been created to repair the `tag` table
for existing relays.
fixes: https://todo.sr.ht/~gheartsfield/nostr-rs-relay/37
