In case of event mode operations where event device can help in atomic
sequence number increment across cores, sequence number need to be
provided by the application instead of being updated in rte_ipsec or the
PMD.

To support this, two new APIs rte_ipsec_pkt_crypto_sqn_assign and
rte_ipsec_pkt_crypto_xprepare are introduced decoupling the seq no update
functionality from the existing rte_ipsec_pkt_crypto_prepare API.
Additionally, a new flag ``RTE_IPSEC_SAFLAG_SQN_ASSIGN_DISABLE`` will be
added to allow disabling of internal sequence number update inside IPsec
library.

Signed-off-by: Aakash Sasidharan <asasidharan@marvell.com>
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