net: coap_client: Fix build with cpp projects

The coap_client.h header won't build if included from c++ file:

error: expected primary-expression before ‘.’ token
  .value[0] = coap_bytes_to_block_size(CONFIG_COAP_CLIENT_BLOCK_SIZE),

Therefore move the actual function implementation to the library C file
to prevent this.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>