[C++] Provide access to acting block length of repeating group via the sbeActingBlockLength() method. Issue aeron-io#968.

Author: mjpt777

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -785,7 +785,6 @@ private void generateGroupClassHeader(
  codecStateNullAssignment);
  }
 
-
  if (null != fieldPrecedenceModel)
  {
  sb.append("\n").append(indent).append("private:");
@@ -808,6 +807,11 @@ private void generateGroupClassHeader(
  indent + " return %2$d;\n" +
  indent + " }\n\n" +
 
+ indent + " SBE_NODISCARD std::uint64_t sbeActingBlockLength() SBE_NOEXCEPT\n" +
+ indent + " {\n" +
+ indent + " return m_blockLength;\n" +
+ indent + " }\n\n" +
+
  indent + " SBE_NODISCARD std::uint64_t sbePosition() const SBE_NOEXCEPT\n" +
  indent + " {\n" +
  indent + " return *m_positionPtr;\n" +