crypto: CLI to change dispatch mode

crypto: CLI to change dispatch mode

This change aims to affect crypto_sw_scheduler behavior,
but all the edits end up in vnet/crypto.

After 9a9604b introduced adaptive mode for crypto dispatch,
the performance of async mode at lower rate got worse.

A work around for CSIT test is done by changing dispatch mode via explicit API call
in https://github.com/FDio/vpp/commit/139aba204780f6cc2845b311820a0b4c47517d02

In this change, the CLI is brought back to allow user to fix the
dispatch mode.

set crypto async dispatch mode <polling|interrupt|adaptive>

Type: improvement
Change-Id: I029e98aa25889eddcf62e75a6c78926cdee862ef
Signed-off-by: Niyaz Murshed <niyaz.murshed@arm.com>