Update protos with Tensorflow data validation schema

[davidheryanto]

What this PR does / why we need it:
This pull request is regarding feature validation in Feast. It updates EntitySpec and FeatureSpec protos with fields to specify constaints for the values. The constraint specs are adapted from Tensorflow metadata schema: https://github.com/tensorflow/metadata/blob/3b9d4114c6856ce18b7819023acb94f07183d348/tensorflow_metadata/proto/v0/schema.proto.

Only presence_constraints, shape_type and domain_info are currently added to EntitySpec and FeatureSpec because it is adequate for the current validation requirement in Feast and to ensure integration with Tensorflow data validation works correctly, before adding more features supported in Tensorflow validation.

Which issue(s) this PR fixes:
Partially addresses #172

Does this PR introduce a user-facing change?:
No, user-facing change with updates in Feast SDK will follow in subsequent pull requests.

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This pull-request has been approved by: davidheryanto

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[zhilingc]

/lgtm

[ches]

I think I've just learned of the Feature Validation RFC, perhaps it could be referenced in the PR description. Although I believe it's only currently accessible to feast-dev—I was wondering if there's a plan to make these publicly readable, when I just found that making this one public was an action item from a December community call 😇

I've taken the liberty to add an agenda point for next call about making these available for feedback earlier.

[woop]

I think I've just learned of the Feature Validation RFC, perhaps it could be referenced in the PR description. Although I believe it's only currently accessible to feast-dev—I was wondering if there's a plan to make these publicly readable, when I just found that making this one public was an action item from a December community call

I've taken the liberty to add an agenda point for next call about making these available for feedback earlier.

Damn we messed up again. Feel free to add comments to the document. We're planning to keep fleshing things out over in the RFC.