Add a new serialized type: STNumber (#5121)

`STNumber` lets objects and transactions contain multiple fields for
quantities of XRP, IOU, or MPT without duplicating information about the
"issue" (represented by `STIssue`). It is a straightforward serialization of
the `Number` type that uniformly represents those quantities.

---------

Co-authored-by: John Freeman <jfreeman08@gmail.com>
Co-authored-by: Howard Hinnant <howard.hinnant@gmail.com>

include/xrpl/protocol/STNumber.h

@@ -0,0 +1,88 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2024 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#ifndef XRPL_PROTOCOL_STNUMBER_H_INCLUDED
+#define XRPL_PROTOCOL_STNUMBER_H_INCLUDED
+
+#include <xrpl/basics/CountedObject.h>
+#include <xrpl/basics/Number.h>
+#include <xrpl/protocol/STBase.h>
+
+#include <ostream>
+
+namespace ripple {
+
+/**
+ * A serializable number.
+ *
+ * This type is-a `Number`, and can be used everywhere that is accepted.
+ * This type simply integrates `Number` with the serialization framework,
+ * letting it be used for fields in ledger entries and transactions.
+ * It is effectively an `STAmount` sans `Asset`:
+ * it can represent a value of any token type (XRP, IOU, or MPT)
+ * without paying the storage cost of duplicating asset information
+ * that may be deduced from the context.
+ */
+class STNumber : public STBase, public CountedObject<STNumber>
+{
+private:
+    Number value_;
+
+public:
+    using value_type = Number;
+
+    STNumber() = default;
+    explicit STNumber(SField const& field, Number const& value = Number());
+    STNumber(SerialIter& sit, SField const& field);
+
+    SerializedTypeID
+    getSType() const override;
+    std::string
+    getText() const override;
+    void
+    add(Serializer& s) const override;
+
+    Number const&
+    value() const;
+    void
+    setValue(Number const& v);
+
+    bool
+    isEquivalent(STBase const& t) const override;
+    bool
+    isDefault() const override;
+
+    operator Number() const
+    {
+        return value_;
+    }
+
+private:
+    STBase*
+    copy(std::size_t n, void* buf) const override;
+    STBase*
+    move(std::size_t n, void* buf) override;
+};
+
+std::ostream&
+operator<<(std::ostream& out, STNumber const& rhs);
+
+}  // namespace ripple
+
+#endif