Use constants instead of hardcoded strings (#330)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR updates some of the hardcoded references to public
constants.


## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Locally
## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No

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

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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-->

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

pkg/model/constants.go

@@ -0,0 +1,34 @@
+package model
+
+// Registry Types - supported package registry types
+const (
+	RegistryTypeNPM   = "npm"
+	RegistryTypePyPI  = "pypi"
+	RegistryTypeOCI   = "oci"
+	RegistryTypeNuGet = "nuget"
+	RegistryTypeMCPB  = "mcpb"
+)
+
+// Registry Base URLs - supported package registry base URLs
+const (
+	RegistryURLNPM    = "https://registry.npmjs.org"
+	RegistryURLPyPI   = "https://pypi.org"
+	RegistryURLDocker = "https://docker.io"
+	RegistryURLNuGet  = "https://api.nuget.org"
+	RegistryURLGitHub = "https://github.com"
+	RegistryURLGitLab = "https://gitlab.com"
+)
+
+// Transport Types - supported remote transport protocols
+const (
+	TransportTypeStreamable = "streamable"
+	TransportTypeSSE        = "sse"
+)
+
+// Runtime Hints - supported package runtime hints
+const (
+	RuntimeHintNPX    = "npx"
+	RuntimeHintUVX    = "uvx"
+	RuntimeHintDocker = "docker"
+	RuntimeHintDNX    = "dnx"
+)