Support fluentbit iso8601 time format for `p_timestamp`

[sp0cket]

fluent-bit.conf

[OUTPUT]
    Name  http
    Match *
    host  127.0.0.1
    header Content-Type application/json
    header_tag X-P-Stream
    format json
    Port 8000
    URI /api/v1/ingest
    json_date_key p_timestamp
    json_date_format iso8601

I got an error Event Error: Stream Writer Failed: Schema error: Fail to merge schema field 'p_timestamp' because the from data_type = Utf8 does not equal Timestamp(Millisecond, None)

And I don't want to have an extra timestamp-like key for the time.

[nitisht]

p_timestamp is an internal field - if you want log event created timestamp to be used for query etc - #683 is the PR for that.

[sp0cket]

This PR seems to be exactly what I want.