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>

tools/validate-examples/main.go

@@ -74,10 +74,26 @@ func runValidation() error {
 			continue
 		}
 
+		// Extract server portion if this is a PublishRequest format
+		serverData := data
+		publishRequestValid := true
+		if dataMap, ok := data.(map[string]any); ok {
+			if server, exists := dataMap["server"]; exists {
+				// This is a PublishRequest format - validate only expected properties exist
+				for key := range dataMap {
+					if key != "server" && key != "x-publisher" {
+						log.Printf("  Invalid PublishRequest property: ❌ %s (only 'server' and optional 'x-publisher' are allowed)", key)
+						publishRequestValid = false
+					}
+				}
+				serverData = server
+			}
+		}
+
 		baseValid := false
 		registryValid := false
 
-		if err := baseSchema.Validate(data); err != nil {
+		if err := baseSchema.Validate(serverData); err != nil {
 			log.Printf("  Validating against schema.json: ❌")
 			log.Printf("    Error: %v", err)
 		} else {
@@ -85,7 +101,7 @@ func runValidation() error {
 			baseValid = true
 		}
 
-		if err := registrySchema.Validate(data); err != nil {
+		if err := registrySchema.Validate(serverData); err != nil {
 			log.Printf("  Validating against registry-schema.json: ❌")
 			log.Printf("    Error: %v", err)
 		} else {
@@ -93,8 +109,8 @@ func runValidation() error {
 			registryValid = true
 		}
 
-		// Only count as validated if both schemas passed
-		if baseValid && registryValid {
+		// Only count as validated if all validations passed
+		if publishRequestValid && baseValid && registryValid {
 			validatedCount++
 		}