auth: Add HTTP-based authentication support to validate domain ownership (#282)

Add HTTP-based authentication to prove domain ownership, similar to
https://github.com/modelcontextprotocol/registry/pull/238's DNS
authentication but using HTTPS well-known URIs instead of DNS TXT
records.

Fixes #280

## Motivation and Context

Some organizations don't have easy access to DNS records, but can easily
deploy web assets. They would therefore prefer to be able to complete a
HTTP challenge rather than a DNS one to authenticate to the registry.

This would in some way mirror ACME challenges: where you can do a DNS-01
or a HTTP-01 challenge.

## How Has This Been Tested?

- Tested locally by stubbing out the HTTP getter
- Added unit tests

## Breaking Changes

None

## 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)
- [ ] 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. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [x] I have added or updated documentation as needed

## Additional context

I concern I had when building this that it might expose us to SSRF
attacks.

I think on reflection this is not a significant problem:
- We only GET a fixed file `.well-known/mcp-registry-auth` (redirects
disallowed) after sanitizing the domain, and then also don't expose the
response to the user. This means the user shouldn't be able to hit
anything very interesting.
- The information they might get is what domains/servers exist in the
network. Our infrastructure is already all open-source, so attackers
aren't getting any additional real info by prodding at the environment
this way.

internal/model/model.go

@@ -10,6 +10,8 @@ const (
 	AuthMethodGitHubOIDC AuthMethod = "github-oidc"
 	// DNS-based public/private key authentication
 	AuthMethodDNS AuthMethod = "dns"
+	// HTTP-based public/private key authentication
+	AuthMethodHTTP AuthMethod = "http"
 	// No authentication - should only be used for local development and testing
 	AuthMethodNone AuthMethod = "none"
 )