Execute self-contained .NET binary

[mikhailshilkov]

Resolves #5334 (other than a binary CLI option, which will move to a new issue)

When the binary option is specified, this PR runs it as dotnet foo.dll for dll files and directly otherwise.

This change looks legit to me but is slightly scary in case I'm missing some other non-dll scenario, so reviews are appreciated.

[EvanBoyle]

Looks good to me. The additional functionality you've added here is basically what we do for Go today:

pulumi/sdk/go/pulumi-language-go/main.go

Lines 45 to 75 in 9ef5c72

	func findProgram(binary string) (*exec.Cmd, error) {
	// we default to execution via `go run`
	// the user can explicitly opt in to using a binary executable by specifying
	// runtime.options.binary in the Pulumi.yaml
	if binary != "" {
	program, err := executable.FindExecutable(binary)
	if err != nil {
	return nil, errors.Wrap(err, "expected to find prebuilt executable")
	}
	return exec.Command(program), nil
	}
	// Fall back to 'go run' style executions
	logging.V(5).Infof("No prebuilt executable specified, attempting invocation via 'go run'")
	program, err := executable.FindExecutable("go")
	if err != nil {
	return nil, errors.Wrap(err, "problem executing program (could not run language executor)")
	}
	cwd, err := os.Getwd()
	if err != nil {
	return nil, errors.Wrap(err, "unable to get current working directory")
	}
	goFileSearchPattern := filepath.Join(cwd, "*.go")
	if matches, err := filepath.Glob(goFileSearchPattern); err != nil || len(matches) == 0 {
	return nil, errors.Errorf("Failed to find go files for 'go run' matching %s", goFileSearchPattern)
	}
	return exec.Command(program, "run", cwd), nil
	}

[justinvp]

Looks reasonable to me.