Support IVF_FLAT backward compatible when cosine

[cydrain]

Issue: #30

In latest IVF_FLAT index, there is a byte to indicate whether the metric type is COSINE in the index header; while the byte indicator not exist in the IVF_FLAT_NM index header.
So when read IVF_FLAT_NM format, knowhere uses the input config json to tell it's COSINE or not, then convert IVF_FLAT_NM format to the new IVF_FLAT format.

[mergify]

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[cydrain]

/kind enhancement

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[cydrain]

/hold

[cydrain]

/unhold

[zhengbuqian]

/lgtm

[cydrain]

/run-e2e

[liliu-z]

/run-e2e