fix(Client): Add DialCloud endpoint

[all-seeing-code]

1. Adds DialCloudEndpoint method
2. Adds deprecation message over DialSlashEndpoint method

This change is 

[manishrjain]

Reviewed 1 of 1 files at r1.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @anurags92)

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client.go, line 91 at r1 (raw file):

// DialCloudEndpoint creates a new TLS connection to a Dgraph Cloud backend
// It requires the backend endpoint as well as the api key

You should add a comment here which explains what to do next.

conn, err := dgo.DialCloud(...)
defer conn.Close()

dg := NewDgraphClient(api.NewDgraphClient(conn))

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client.go, line 92 at r1 (raw file):

// DialCloudEndpoint creates a new TLS connection to a Dgraph Cloud backend
// It requires the backend endpoint as well as the api key
func DialCloudEndpoint(endpoint, key string) (*grpc.ClientConn, error) {

DialCloud