Allow for vendor extensions on servers (#298)

<!-- Provide a brief summary of your changes -->
This PR serves as a placeholder and brings together all of the changes
related to supporting vendor extensions passed through the API. The idea
is to have a single place to track and review the work as it comes
together (even if it’s still a bit of a work in progress).

**The changes in this PR cover:**
* Updates on the data model
* Updates on both databases (in-memory and postgresql), added a new
table to PSQL for tracking the extensions leaving the server table only
for the clean server.json properties
* Updates the seed.json file (thus the big line change)
* Adds a seed-migrate tool to help migrating an existing seed file to
the new format
* Updates the seed import mechanism to support this
* Updates the existing tests - unit, integration and validation tests
* Adds the publish handler to the OpenAPI spec (it was missing)
* Fixes linting errors
* Updates on the API side on (publish, list, get) to support the new
format of `ServerResponse` -
```
{
  "server": {},
  "x-io.modelcontextprotocol.registry": {}, /*not available for publish since this gets populated by the registry*/
  "x-publisher": {}
}
```

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
Motivated by the discussions in:
https://github.com/modelcontextprotocol/registry/discussions/284

## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
Locally and by running all of the tests

## Breaking Changes
<!-- Will users need to update their code or configurations? -->
Yes, it changes both the API and the format of the seed file
 
## 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)
- [x] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

---

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>
Co-authored-by: adam jones <domdomegg+git@gmail.com>

internal/database/database.go

@@ -16,14 +16,14 @@ var (
 	ErrInvalidVersion = errors.New("invalid version: cannot publish older version after newer version")
 )
 
-// Database defines the interface for database operations on MCPRegistry entries
+// Database defines the interface for database operations with extension wrapper architecture
 type Database interface {
-	// List retrieves all MCPRegistry entries with optional filtering
-	List(ctx context.Context, filter map[string]any, cursor string, limit int) ([]*model.Server, string, error)
-	// GetByID retrieves a single ServerDetail by it's ID
-	GetByID(ctx context.Context, id string) (*model.ServerDetail, error)
-	// Publish adds a new ServerDetail to the database
-	Publish(ctx context.Context, serverDetail *model.ServerDetail) error
+	// List retrieves all ServerRecord entries with optional filtering
+	List(ctx context.Context, filter map[string]any, cursor string, limit int) ([]*model.ServerRecord, string, error)
+	// GetByID retrieves a single ServerRecord by its ID
+	GetByID(ctx context.Context, id string) (*model.ServerRecord, error)
+	// Publish adds a new server to the database with separated server.json and extensions
+	Publish(ctx context.Context, serverDetail model.ServerDetail, publisherExtensions map[string]interface{}) (*model.ServerRecord, error)
 	// ImportSeed imports initial data from a seed file
 	ImportSeed(ctx context.Context, seedFilePath string) error
 	// Close closes the database connection