proxy: add support for user defined metadata in model configs (#333)

Changes: 

- add Metadata key to ModelConfig
- include metadata in /v1/models under meta.llamaswap key
- add recursive macro substitution into Metadata
- change macros at global and model level to be any scalar type

Note: 

This is the first mostly AI generated change to llama-swap. See #333 for notes about the workflow and approach to AI going forward.

proxy/proxymanager.go

@@ -370,6 +370,13 @@ func (pm *ProxyManager) listModelsHandler(c *gin.Context) {
 			record["description"] = desc
 		}
 
+		// Add metadata if present
+		if len(modelConfig.Metadata) > 0 {
+			record["meta"] = gin.H{
+				"llamaswap": modelConfig.Metadata,
+			}
+		}
+
 		data = append(data, record)
 	}