Adding char const* buffer() const noexcept; method cpp

In order to be able to pass a const reference of the SBE Message and access the underlying buffer 
consider adding method:

```C
char const* buffer() const SBE_NOEXCEPT;

char const* buffer() const SBE_NOEXCEPT
{
    return m_buffer;
} 
```

The generator only generates a non-const version:

```C
char* buffer() SBE_NOEXCEPT
{
     return m_buffer;
}
```