Split Sendable into NT and non-NT portions (#3432)

The non-NT portion has been moved to wpiutil.
The NT portion has been moved to ntcore (as NTSendable).

SendableBuilder similarly split and moved.

SendableRegistry moved to wpiutil.

In C++, SendableHelper also moved to wpiutil.

This enables use of Sendable from wpimath and also enables
moving several classes from wpilib to wpimath.

ntcore/src/main/native/include/networktables/NTSendable.h

@@ -0,0 +1,28 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <wpi/sendable/Sendable.h>
+
+namespace nt {
+
+class NTSendableBuilder;
+
+/**
+ * Interface for NetworkTable Sendable objects.
+ */
+class NTSendable : public wpi::Sendable {
+ public:
+  /**
+   * Initializes this Sendable object.
+   *
+   * @param builder sendable builder
+   */
+  virtual void InitSendable(NTSendableBuilder& builder) = 0;
+
+  void InitSendable(wpi::SendableBuilder& builder) override;
+};
+
+}  // namespace nt

ntcore/src/main/native/include/networktables/NTSendableBuilder.h

@@ -0,0 +1,65 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#pragma once
+
+#include <functional>
+#include <memory>
+#include <string_view>
+
+#include <wpi/sendable/SendableBuilder.h>
+
+#include "networktables/NetworkTable.h"
+#include "networktables/NetworkTableEntry.h"
+#include "networktables/NetworkTableValue.h"
+
+namespace nt {
+
+class NTSendableBuilder : public wpi::SendableBuilder {
+ public:
+  /**
+   * Set the function that should be called to update the network table
+   * for things other than properties.  Note this function is not passed
+   * the network table object; instead it should use the entry handles
+   * returned by GetEntry().
+   *
+   * @param func    function
+   */
+  virtual void SetUpdateTable(std::function<void()> func) = 0;
+
+  /**
+   * Add a property without getters or setters.  This can be used to get
+   * entry handles for the function called by SetUpdateTable().
+   *
+   * @param key   property name
+   * @return Network table entry
+   */
+  virtual NetworkTableEntry GetEntry(std::string_view key) = 0;
+
+  /**
+   * Add a NetworkTableValue property.
+   *
+   * @param key     property name
+   * @param getter  getter function (returns current value)
+   * @param setter  setter function (sets new value)
+   */
+  virtual void AddValueProperty(
+      std::string_view key, std::function<std::shared_ptr<Value>()> getter,
+      std::function<void(std::shared_ptr<Value>)> setter) = 0;
+
+  /**
+   * Get the network table.
+   * @return The network table
+   */
+  virtual std::shared_ptr<NetworkTable> GetTable() = 0;
+
+  /**
+   * Gets the kind of backend being used.
+   *
+   * @return Backend kind
+   */
+  BackendKind GetBackendKind() const override;
+};
+
+}  // namespace nt