[WIP] Add golang SDK

[zhilingc]

Addresses #260 .

Some considerations:

• The protobuf defined request and response payloads are not very user friendly, so we need wrappers around them
• Performance is of great importance for this SDK so we want to avoid reflection as much as possible
• Most feast usecases deal with single-type arrays of features, but there are usecases that involve mixed type outputs, so we have to support both

[feast-ci-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zhilingc

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[zhilingc]

Usage example for the current iteration:

client, err := NewGrpcClient("localhost", 6565)
if err != nil {
	panic(err)
}
ctx := context.Background()
response, err := client.GetOnlineFeatures(ctx, &OnlineFeaturesRequest{
	Features:      []string{"fs:1:feature1", "fs:1:feature2"},
	MaxAgeSeconds: 100,
	Entities: []Row{
		{"s2id": Int64Val(1)},
		{"s2id": Int64Val(2)},
	},
})
if err != nil {
	panic(err)
}

// just retrieve rows
// returns map[string]*types.Value
_ = response.Rows()

// use convenience functions to get arrays
// returns [][]int64
_, err = response.Int64Arrays([]string{"fs:1:feature1", "fs:1:feature2"}, []int64{-1, -1})

[woop]

Overall it looks good! I like the construction of the request

[feast-ci-bot]

@zhilingc: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
integration-test	4821cf5	link	/test integration-test
unit-test-cli	4821cf5	link	/test unit-test-cli
unit-test-ingestion	4821cf5	link	/test unit-test-ingestion

Full PR test history

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