Introduce v0.1 version of the API (#658)

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

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
The following PR adds a new v0.1 API version. 

The goal is to mark v0.1 as a "freeze" point while keeping v0 as the
moving target for new features so integrators can start building on top
of something stable(r).

Details:
* This change is quite simple - we re-register the same handlers twice,
once as `v0` and once as `v0.1`
* Going forward, we'll keep v0.1 frozen while we add changes either on
top of both (non-breaking) or we'll have to branch if breaking changes
are necessary
* We all acknowledge this is not exactly best-practice, but it gives us
a way to gather feedback and allow clients to start integrating with the
API without breaking them (**note** this implies a follow up PR that
would branch the handlers used in v0.1).
* For integrators/clients: please provide your feedback - what works
well, what doesn't so we can take this into account going towards GA.

## How Has This Been Tested?
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tested? -->

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
No, this just adds a new API version alongside the existing one.

## 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)
- [ ] 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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-->
Fixes: #649

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

internal/api/handlers/v0/auth/github_at.go

@@ -42,15 +42,15 @@ func (h *GitHubHandler) SetBaseURL(url string) {
 	h.baseURL = url
 }
 
-// RegisterGitHubATEndpoint registers the GitHub access token authentication endpoint
-func RegisterGitHubATEndpoint(api huma.API, cfg *config.Config) {
+// RegisterGitHubATEndpoint registers the GitHub access token authentication endpoint with a custom path prefix
+func RegisterGitHubATEndpoint(api huma.API, pathPrefix string, cfg *config.Config) {
 	handler := NewGitHubHandler(cfg)
 
 	// GitHub token exchange endpoint
 	huma.Register(api, huma.Operation{
-		OperationID: "exchange-github-token",
+		OperationID: "exchange-github-token" + pathPrefix,
 		Method:      http.MethodPost,
-		Path:        "/v0/auth/github-at",
+		Path:        pathPrefix + "/auth/github-at",
 		Summary:     "Exchange GitHub OAuth access token for Registry JWT",
 		Description: "Exchange a GitHub OAuth access token for a short-lived Registry JWT token",
 		Tags:        []string{"auth"},