Make NostrDB (and related) code build under the new extension target as well.

This change includes several source files related to NostrDB into the extension target as well, so that we can use it from that context (and thus enable more advanced push notification formatting and suppression)

To make this change possible, I had to split some source files as well as to move some functions to different files, to ensure we don't have to pull too much unnecessary code into the extension.

Testing
-------

PASS

Device: iPhone 15 Pro simulator
iOS: 17.0.1
Damus: This commit
Test steps:
1. Build DamusNotificationService. Should succeed. PASS
2. Build Damus (the app). PASS
3. Run app, scroll around some notes, go to a few different views, post a note. Should work as normal. PASS

nostrdb/NdbNote.swift

@@ -7,7 +7,12 @@
 
 import Foundation
 import NaturalLanguage
+import CommonCrypto
+import secp256k1
+import secp256k1_implementation
+import CryptoKit
 
+let MAX_NOTE_SIZE: Int = 2 << 18
 
 struct NdbStr {
     let note: NdbNote
@@ -42,12 +47,7 @@ class NdbNote: Encodable, Equatable, Hashable {
     let note: UnsafeMutablePointer<ndb_note>
 
     // cached stuff (TODO: remove these)
-    private var _event_refs: [EventRef]? = nil
     var decrypted_content: String? = nil
-    private var _blocks: Blocks? = nil
-    private lazy var inner_event: NdbNote? = {
-        return NdbNote.owned_from_json_cstr(json: content_raw, json_len: content_len)
-    }()
 
     init(note: UnsafeMutablePointer<ndb_note>, owned_size: Int?, key: NoteKey?) {
         self.note = note
@@ -252,8 +252,7 @@ class NdbNote: Encodable, Equatable, Hashable {
     }
 }
 
-
-// NostrEvent compat
+// Extension to make NdbNote compatible with NostrEvent's original API
 extension NdbNote {
     var is_textlike: Bool {
         return kind == 1 || kind == 42 || kind == 30023
@@ -275,19 +274,6 @@ extension NdbNote {
         return parse_note_content(content: .init(note: self, keypair: keypair))
     }
 
-    func get_inner_event(cache: EventCache) -> NostrEvent? {
-        guard self.known_kind == .boost else {
-            return nil
-        }
-
-        if self.content_len == 0, let id = self.referenced_ids.first {
-            // TODO: raw id cache lookups
-            return cache.lookup(id)
-        }
-
-        return self.inner_event
-    }
-
     // TODO: References iterator
     public var referenced_ids: References<NoteId> {
         References<NoteId>(tags: self.tags)
@@ -322,11 +308,7 @@ extension NdbNote {
     }
 
     func event_refs(_ keypair: Keypair) -> [EventRef] {
-        if let rs = _event_refs {
-            return rs
-        }
         let refs = interpret_event_refs_ndb(blocks: self.blocks(keypair).blocks, tags: self.tags)
-        self._event_refs = refs
         return refs
     }
 
@@ -339,11 +321,7 @@ extension NdbNote {
     }
 
     func blocks(_ keypair: Keypair) -> Blocks {
-        if let bs = _blocks { return bs }
-
-        let blocks = get_blocks(keypair: keypair)
-        self._blocks = blocks
-        return blocks
+        return get_blocks(keypair: keypair)
     }
 
     // NDBTODO: switch this to operating on bytes not strings
@@ -448,3 +426,15 @@ extension NdbNote {
         return Date.now.timeIntervalSince(event_date)
     }
 }
+
+func hex_encode(_ data: Data) -> String {
+    var str = ""
+    for c in data {
+        let c1 = hexchar(c >> 4)
+        let c2 = hexchar(c & 0xF)
+
+        str.append(Character(Unicode.Scalar(c1)))
+        str.append(Character(Unicode.Scalar(c2)))
+    }
+    return str
+}