Implement NostrNetworkManager and UserRelayListManager

This commit implements a new layer called NostrNetworkManager,
responsible for managing interactions with the Nostr network, and
providing a higher level API that is easier and more secure to use for
the layer above it.

It also integrates it with the rest of the app, by moving RelayPool and PostBox
into NostrNetworkManager, along with all their usages.

Changelog-Added: Added NIP-65 relay list support
Changelog-Changed: Improved robustness of relay list handling
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damusTests/Mocking/MockDamusState.swift

@@ -27,8 +27,7 @@ func generate_test_damus_state(
     }()
     
     let mutelist_manager = MutelistManager(user_keypair: test_keypair)
-    let damus = DamusState(pool: pool,
-                           keypair: test_keypair,
+    let damus = DamusState(keypair: test_keypair,
                            likes: .init(our_pubkey: our_pubkey),
                            boosts: .init(our_pubkey: our_pubkey),
                            contacts: .init(our_pubkey: our_pubkey), mutelist_manager: mutelist_manager,
@@ -43,8 +42,6 @@ func generate_test_damus_state(
                            drafts: .init(),
                            events: .init(ndb: ndb),
                            bookmarks: .init(pubkey: our_pubkey),
-                           postbox: .init(pool: pool),
-                           bootstrap_relays: .init(),
                            replies: .init(our_pubkey: our_pubkey),
                            wallet: .init(settings: settings),
                            nav: .init(),