Add a replies table to store reply events

[im-adithya]

Currently we only store the reply ID in the NostrEvent table (which are basically the received events of Kind NIP_47_REQUEST) after publishing a reply. Now that we are using an SQLite DB with #217, it's best to add a table to store these reply events early on to avoid having to migrate later.

Additional reasons are also mentioned here: #230

This also helps us in multi_methods (#218) as there we don't just have one reply per NostrEvent but multiple.

[rolznz]

@im-adithya we need to do this as part of #217 right? I will add a TODO there

EDIT: it needs to be done as part of #218 right?

[im-adithya]

Done here: getAlby#3