Introduce protocol abstraction interfaces

[smdn]

Description

Introduce an interface IMuninProtocolHandler to abstract the implementation of the data exchange protocol between munin master and munin node.

This PR creates a new namespace Smdn.Net.MuninNode.Protocol and introduces the interfaces IMuninProtocolHandlerFactory and IMuninProtocolHandler.
The munin protocol handling that has been implemented in NodeBase will be moved and separated to a new class MuninProtocolHandler that implements IMuninProtocolHandler.

In addition, an interface IMuninNodeProfile will be introduced to reduce the consolidation between NodeBase and IMuninProtocolHandler.

The outlines of the APIs to be introduced by this change are as follows:

+namespace Smdn.Net.MuninNode.Protocol {
+  public interface IMuninProtocolHandler {
+    ValueTask HandleCommandAsync(IMuninNodeClient client, ReadOnlySequence<byte> commandLine, CancellationToken cancellationToken);
+    ValueTask HandleTransactionEndAsync(IMuninNodeClient client, CancellationToken cancellationToken);
+    ValueTask HandleTransactionStartAsync(IMuninNodeClient client, CancellationToken cancellationToken);
+  }
+
+  public interface IMuninProtocolHandlerFactory {
+    ValueTask<IMuninProtocolHandler> CreateAsync(IMuninNodeProfile profile, CancellationToken cancellationToken);
+  }
+
+  public static class MuninProtocolHandlerFactory {
+    public static IMuninProtocolHandlerFactory Default { get; }
+  }
+
+  public interface IMuninNodeProfile {
+    Encoding Encoding { get; }
+    string HostName { get; }
+    IPluginProvider PluginProvider { get; }
+    string Version { get; }
+  }
+}

This change will improve the independence, extensibility, testability, and maintainability of the protocol implementation.
At the same time, reorganize the code base to improve protocol handling in the future.

This PR also fixes race condition due to use of single IBufferWriter while sending response.

Considerations

In this PR, the term transaction is used instead of the term session, following the example in the Munin's official document.
In the future, the terminology used with names such as INodeSessionCallback needs to be consistent with transaction.