Initial KV API concept

Core/Option/OptionItem.cs

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+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading.Tasks;
+
+namespace Kruzya.TelegramBot.Core.Option;
+
+public class OptionItem<T> : IOptionItem<T>
+{
+    public IOptionKey Key { get; }
+
+    public T Value
+    {
+        get => _value;
+
+        set
+        {
+            _value = value;
+            _isChanged = true;
+        }
+    }
+
+    public bool IsChanged => _isChanged;
+
+    private T _value;
+    private bool _isChanged;
+
+    public OptionItem(IOptionKey key, T value)
+    {
+        Key = key;
+        Value = value;
+    }
+
+    public async Task LoadAsync(IOptionStorageAdapter adapter)
+    {
+        var savedValue = await adapter.LoadValueAsync(Key);
+        if (savedValue == null)
+        {
+            return;
+        }
+
+        // This property changed when we're loaded default value. Reset here.
+        _isChanged = false;
+        _value = JsonSerializer.Deserialize<T>(new ReadOnlySpan<byte>(savedValue));
+    }
+
+    public async Task SaveAsync(IOptionStorageAdapter adapter)
+    {
+        var data = JsonSerializer.SerializeToUtf8Bytes(Value, new JsonSerializerOptions { WriteIndented = false, 
+            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull });
+        await adapter.SaveValueAsync(Key, data);
+    }
+}