add function to tile package for reading metadata from a tanzunet product file

[crhntr]

This will be helpful for making kiln test tile revisions against recently released tiles.

I intend to use this on a few upcoming features but am adding it now because this function is a helpful utility that TAS Operability may get value from.

I intentionally did not use the pivnet package because we use various versions of that module in this package. Once #318 is merged, we may want to re-evaluate this API. As it is, this implementation might work with Artifactory. I expect a future utility function to call this and pass in the appropriate information from a pivnet.ProductFile (ppet has an example of how to do this if the test comment is not enough).

This test change also switches from "gomega" to "testify/assert" for the tile package. We have talked about whether or not to do this for the whole project. For now, I think we should converge to one set of tools per package. Each package may have different needs served better by Gomega/Ginkgo, godog, standard library alone, or standard library + testify.

Example

// create http.Request from pivnet.ProductFile
downloadLink, _ := productFile.DownloadLink()
ctx := context.Background()
req, _ := http.NewRequestWithContext(ctx, http.MethodGet, downloadLink, nil)
// on a real request you need to set "Authorization" and "User-Agent" headers

metadataBytes, _ := tile.ReadMetadataFromServer(http.DefaultClient, req)
// see test for running example

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