add feature for write/read data reusing the buffer passed in

[JacobBarthelmeh]

Part of the PIC18f effort to reduce memory used. This avoids having the application allocate a plain text buffer to then pass to wolfSSL and have copied over to an internal 'send' buffer. And on the receiving side avoids having an internal 'recv' buffer in addition to the applications buffer for holding plain text data received. Helping to reduce the overall memory used.

i.e. Sending a 50 byte data message before would use 50 bytes in an application buffer created, plus 50 + record header + Mac output buffer in wolfSSL internally. Now it's one 50 + record header + Mac buffer created in the application and passed to wolfSSL that gets reused.