Add time_key and time_key_format config options

[PettitWesley]

Issue #, if available:
aws/amazon-kinesis-firehose-for-fluent-bit#9

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[PettitWesley]

Ran integ tests, then tested the time key change with this config:

[OUTPUT]
    Name   kinesis
    Match  *
    region us-west-2
    stream integ-test-fluent-bit-kinesisStream-HPCEW6APH6EZ
    time_key  time
    time_key_format %Y-%m-%dT%H:%M:%S%z

It works.