feat(config): add support for disabling OpenAI compatibility providers

- Introduced a `Disabled` flag to OpenAI compatibility configurations.
- Updated routing, auth selection, and API handling logic to respect the `Disabled` state.
- Extended relevant APIs, YAML configurations, and data structures to include the `Disabled` field.
- Adjusted all relevant loops and filters to skip disabled providers.

Closes: #3060 #3059 #2977

internal/api/server.go

@@ -1100,6 +1100,9 @@ func (s *Server) UpdateClients(cfg *config.Config) {
 	openAICompatCount := 0
 	for i := range cfg.OpenAICompatibility {
 		entry := cfg.OpenAICompatibility[i]
+		if entry.Disabled {
+			continue
+		}
 		openAICompatCount += len(entry.APIKeyEntries)
 	}