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:
#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 <[email protected]>
Co-authored-by: adam jones <[email protected]>

Author: rdimitrov

cmd/registry/main.go

@@ -46,7 +46,7 @@ func main() {
 	// Initialize services based on environment
 	switch cfg.DatabaseType {
 	case config.DatabaseTypeMemory:
-		db = database.NewMemoryDB(map[string]*model.Server{})
+		db = database.NewMemoryDB(map[string]*model.ServerDetail{})
 		registryService = service.NewRegistryServiceWithDB(db)
 	case config.DatabaseTypePostgreSQL:
 		// Use PostgreSQL for real registry service