Add day 1

Days/Day.cs

@@ -1,3 +1,5 @@
+using System.Reflection;
+
 namespace AdventOfCode.Days;
 
 public abstract class Day
@@ -5,9 +7,19 @@ public abstract class Day
     public abstract int Number { get; }
     public abstract string Name { get; }
 
+    protected string Input { get; private set; } = null!;
+
     public abstract void RunPart1(bool display = true);
     public abstract void RunPart2(bool display = true);
 
+    public void ReadInput()
+    {
+        using var inputStream = Assembly.GetExecutingAssembly()
+            .GetManifestResourceStream($"AdventOfCode.Inputs.Day{Number}.txt")!;
+
+        Input = new StreamReader(inputStream).ReadToEnd();
+    }
+
     public override string ToString()
     {
         return $"Day {Number}: {Name}";