support humantime_serde in flush-period field

[fraidev]

It would be nice flush-period field of consumer offset config also support humantime_serde.

now:

apiVersion: 0.2.0
meta:
  consumer:
    partition: all
    id: $CONNECTOR_NAME
    offset:
      strategy: auto
      start: beginning
      flush-period:
        secs: 10
        nanos: 0

expected:

apiVersion: 0.2.0
meta:
  consumer:
    partition: all
    id: $CONNECTOR_NAME
    offset:
      strategy: auto
      start: beginning
      flush-period: 10s