Bluetooth: ISO: Add sequence number to ISO data packets

The sequence number is needed in the appliaction layer to detect lost
packets in the ISO stream in cases where the timestamp is not included.
(Sequence number is mandatory to include where timestamp is optional).
The API for the public metadata have been moved to a public header file.

As the size of the ISO meta data exceeds the default 4 octets net_buf
user_data the public ISO metadata have been moved into a seperate array.
The internal metadata is still stored in net_buf user_data.

This also fixed the user_data overflow on 32 bit systems, caused by
writing the ts into user_data on index 4 to 7, which is outside the 4
allocated bytes.

Signed-off-by: Casper Bonde <casper_bonde@bose.com>