net: coap_client: allow application to add block2 option to requests

Allow an application to add a Block2 option to an initial request for a
resource. For any subsequent requests as part of a blockwise transfer,
drop the application-added Block2 option since the coap_client must
append a Block2 option with updated NUM and SZX fields based on the
server response.

Signed-off-by: Matt Rodgers <mrodgers@witekio.com>