Add matrix-based testing

[hallipr]

No description provided.

[heaths]

Apart from the one comment, I don't see anything obviously wrong, but @benbp might be a better reviewer here. And, at least as of when I wrote this, all the pipelines failed so there's something clearly wrong but I couldn't say what from what I see. But I don't see anything to hold the PR up.

[benbp]

Suggested change

	SparseCheckoutPaths:
	- /*

I don't think you need this? It's just for checking out files that may be necessary for matrix generation outside of eng+service directory.

[hallipr]

I think for PR matrix generation, I'll be back to needing it because I use cargo metadata in LanguageSettings to get package metadata

[hallipr]

So ci.yml will need all the code because any of it may need to be built. Depending on how cargo handles partial workspaces, we may still need all the cargo.tomls to do the cargo metadata call even when working with a single service directory.

[benbp]

The way the matrix is set up now, you could specify it with all instead. Though perhaps you have it with NonSparseParameters to handle when more matrix parameters are added?

Suggested change

	default:
	- Name: rust_ci_test_base
	Path: eng/pipelines/templates/stages/platform-matrix.json
	Selection: sparse
	NonSparseParameters: RustToolchainName
	GenerateVMJobs: true
	default:
	- Name: rust_ci_test_base
	Path: eng/pipelines/templates/stages/platform-matrix.json
	Selection: all
	GenerateVMJobs: true

[hallipr]

I wasn't sure how selection: all would interact with sparse coming from the PR matrix generation