Add registry validators (#350)

Fixes #96

## Motivation and Context

See #96

## How Has This Been Tested?

Has some unit and integration tests, still needs proper testing of the
different registry types

## Breaking Changes

Yes, new package uploads will need extra verification

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

internal/validators/registries/testutils_test.go

@@ -0,0 +1,32 @@
+package registries_test
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+	"fmt"
+)
+
+func generateRandomPackageName() string {
+	bytes := make([]byte, 16)
+	if _, err := rand.Read(bytes); err != nil {
+		// Fallback to a static name if crypto/rand fails
+		return "nonexistent-pkg-fallback"
+	}
+	return fmt.Sprintf("nonexistent-pkg-%s", hex.EncodeToString(bytes))
+}
+
+func generateRandomNuGetPackageName() string {
+	bytes := make([]byte, 16)
+	if _, err := rand.Read(bytes); err != nil {
+		return "NonExistent.Package.Fallback"
+	}
+	return fmt.Sprintf("NonExistent.Package.%s", hex.EncodeToString(bytes)[:16])
+}
+
+func generateRandomImageName() string {
+	bytes := make([]byte, 16)
+	if _, err := rand.Read(bytes); err != nil {
+		return "nonexistent-image-fallback"
+	}
+	return fmt.Sprintf("nonexistent-image-%s", hex.EncodeToString(bytes)[:16])
+}