Add ContactPubkeyProvider

Sources/ElixxirDAppsSDK/ContactPubkeyProvider.swift

+import Bindings
+
+public struct ContactPubkeyProvider {
+  public var get: (Data) throws -> Data
+
+  public func callAsFunction(contact: Data) throws -> Data {
+    try get(contact)
+  }
+}
+
+extension ContactPubkeyProvider {
+  public static let live = ContactPubkeyProvider { contact in
+    var error: NSError?
+    let pubkey = BindingsGetPubkeyFromContact(contact, &error)
+    if let error = error {
+      throw error
+    }
+    guard let pubkey = pubkey else {
+      fatalError("BindingsGetPubkeyFromContact returned `nil` without providing error")
+    }
+    return pubkey
+  }
+}
+
+#if DEBUG
+extension ContactPubkeyProvider {
+  public static let failing = ContactPubkeyProvider { _ in
+    struct NotImplemented: Error {}
+    throw NotImplemented()
+  }
+}
+#endif