Add Unix Timestamp value type

[MattDelac]

What this PR does / why we need it:
As discussed in Slack, so far, we don't want to support datetime dtypes in general. Therefore, so far we would need to convert any datetime data types into a unix timestamp and store it as an INT in feast.

While converting in unix timestamp is not an issue, it makes it hard to differentiate the features that are int vs timestamp when looking at a FeatureView

Thus this PR aims to introduce a new datatype "Unix Timestamp" that beyond the scene would be an INT. For the end user, this will make it easier understand a FeatureView.
It also add a potential possibility to do things differently with Unix Timestamp features at retrieval time.

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

Add UnixTimestamp valueType that is an Int64 under the hood

[feast-ci-bot]

Hi @MattDelac. Thanks for your PR.

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[woop]

/ok-to-test

[woop]

/kind feature

[woop]

Hey @MattDelac, thanks for the contribution!

Overall it looks good to me. One thing that is a glaring omissions (not with your PR but with our code) is that we're not testing all the different types. Regardless, we can add that test a bit later. I've added a note to add a test here.

I think the only thing missing here is an update to our documentation. You may need to update the specs to document the new type: https://github.com/feast-dev/feast/tree/master/docs/specs

[MattDelac]

One thing that is a glaring omissions (not with your PR but with our code) is that we're not testing all the different types.

Yes that was my surprise that none of the tests had to be changed

I've added a note to add a test here.

Sounds good. I let you fix it later 👌

I think the only thing missing here is an update to our documentation

Good catch ! Will look into the doc

[MattDelac]

I think the only thing missing here is an update to our documentation. You may need to update the specs to document the new type: https://github.com/feast-dev/feast/tree/master/docs/specs

Done. The docs are now up to date. Let me know if I missed another one.

[woop]

/lgtm

[woop]

/lgtm

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: MattDelac, woop

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[MattDelac]

/lgtm

[feast-ci-bot]

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/lgtm

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[woop]

/lgtm