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/config/config_posix_test.go

@@ -163,7 +163,7 @@ groups:
 	expected := Config{
 		LogLevel:  "info",
 		StartPort: 5800,
-		Macros: map[string]string{
+		Macros: MacroList{
 			"svr-path": "path/to/server",
 		},
 		Hooks: HooksConfig{