Control: Add Failover interface

Create an interface to start a failover, where a failover is the
mechanism where one entity takes over the operation of another.

The first use will be to failover to

Control: Add Failover interface

Create an interface to start a failover, where a failover is the
mechanism where one entity takes over the operation of another.

The first use will be to failover to another BMC, using the design at
https://gerrit.openbmc.org/c/openbmc/docs/+/70233, though the intent is
the interface can also be used for other sorts of failovers.

Change-Id: I32789f6b8f01d1baed95d387e6815a797d5b45b8
Signed-off-by: Matt Spinler <spinler@us.ibm.com>

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