net: tftp: Add client context

Use client context to seperate buffer usage.
Use new `TFTP_EVT_DATA` event to send data to application.
Use new `TFTP_EVT_ERROR` event to report error to application.
Update `tftp_get()` and `tftp_put()` API to use the client context.

Signed-off-by: Jun Qing Zou <jun.qing.zou@nordicsemi.no>