Bluetooth: Host: Introduce bt_hci_cmd_alloc()

Introduce a new bt_hci_cmd_alloc() API which only allocates a command
buffer and reserves sufficient headroom for H:4 and command headers, but
doesn't actually encode any of those headers into the buffer. Additionally,
modify bt_hci_cmd_send() and bt_hci_cmd_send_sync() so that they handle
such buffers and perform the header encoding correctly. To avoid
duplication, bt_hci_cmd_send_sync() now builds upon (i.e. calls internally)
the bt_hci_cmd_send() API.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>