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/model_config.go

@@ -31,6 +31,10 @@ type ModelConfig struct {
 	// Macros: see #264
 	// Model level macros take precedence over the global macros
 	Macros MacroList `yaml:"macros"`
+
+	// Metadata: see #264
+	// Arbitrary metadata that can be exposed through the API
+	Metadata map[string]any `yaml:"metadata"`
 }
 
 func (m *ModelConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {