Finish the implementation of NIP-01

Our implementation of NIP-01 is not very complete, the following list of items is missing (at least).

Filtering is only implemented to match on kind in `filter_events` (in `src/clienthandler.rs`), spec says “at least one of the array’s values must match the relevant field in an event for the condition itself to be considered a match” so we should match all filter attributes (`ids`, `authors`, `kinds`, `#e`, `#p`, `since`, `until`).

The spec says “all conditions of a filter that are specified must match for an event for it to pass the filter, i.e., multiple conditions are interpreted as `&&` conditions”, so when we parse an `Event` against `Filter` the event should satisfy the concatenation of all the attributes.

We do not enforce the `limit` property, this can be done at reception of `ClientMessage::Req` though here we might need a notion of a store first, see "TODO: replay stored events when there is a store”

There is no implementation of metadata assigned to our `NostrClient` (in `src/clienthandler.rs`) as a result of calling `setmetadata` in the sample.

There is no implementation of recommendation assigned to our `NostrClient` (in `src/clienthandler.rs`) as a result of calling `recommendserver` in the sample.

Our `opensubscription` command in the sample do not take as argument all the filter attributes (`ids`, `authors`, `kinds`, `#e`, `#p`, `since`, `until`), neither can accept an array of filters.

Can be splitted in multiple PRs as some of the items might be buffy.