The Cord::GetCustomAppendBuffer() method provides the same functionality as Cord::GetAppendBuffer(), except that callers can specify a custom block size/limit if the method returns a newly allocated buffer.

In other words: Cord::GetAppendBuffer() defaults to CordBuffer::CreateWithDefaultLimit(), Cord::GetCustomAppendBuffer() defaults to CordBuffer::CreateWithCustomLimit().

PiperOrigin-RevId: 461231989
Change-Id: I5c03f31139d9b068feee1bea76d59e1c5e30ef07