feat: Add creds for GitHub auth, add "none" auth to publisher (#276)

## Motivation and Context

- Adds credentials for the new MCP Registry Login GitHub Apps (local,
staging, prod)
- Adds support for "none" auth to publisher

## How Has This Been Tested?

- Tested publisher workflows locally
- Unit and integration tests passing

## 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

---

Merging based on Tadas's approval review, and only making minor docs changes on top of that.

README.md

@@ -204,20 +204,7 @@ The API is documented using Swagger/OpenAPI. This page provides a complete refer
 
 ## Configuration
 
-The service can be configured using environment variables:
-
-| Variable | Description | Default |
-|----------|-------------|---------|
-| `MCP_REGISTRY_APP_VERSION`           | Application version | `dev` |
-| `MCP_REGISTRY_DATABASE_TYPE`         | Database type | `mongodb` |
-| `MCP_REGISTRY_COLLECTION_NAME`       | MongoDB collection name | `servers_v2` |
-| `MCP_REGISTRY_DATABASE_NAME`         | MongoDB database name | `mcp-registry` |
-| `MCP_REGISTRY_DATABASE_URL`          | MongoDB connection string | `mongodb://localhost:27017` |
-| `MCP_REGISTRY_GITHUB_CLIENT_ID`      | GitHub App Client ID |  |
-| `MCP_REGISTRY_GITHUB_CLIENT_SECRET`  | GitHub App Client Secret |  |
-| `MCP_REGISTRY_LOG_LEVEL`             | Log level | `info` |
-| `MCP_REGISTRY_SEED_FROM`             | Path or URL to import seed data (supports local files and HTTP URLs) | `data/seed.json` |
-| `MCP_REGISTRY_SERVER_ADDRESS`        | Listen address for the server | `:8080` |
+The service can be configured using environment variables. See [.env.example](./.env.example) for details.
 
 ## Pre-built Docker Images