feat: support request_header key source (#4903)

* feat: support request_header key source in backend and settings UI

* feat: support request_header channel affinity source

setting/operation_setting/channel_affinity_setting.go

@@ -3,7 +3,7 @@ package operation_setting
 import "github.com/QuantumNous/new-api/setting/config"
 
 type ChannelAffinityKeySource struct {
-	Type string `json:"type"` // context_int, context_string, gjson
+	Type string `json:"type"` // context_int, context_string, request_header, gjson
 	Key  string `json:"key,omitempty"`
 	Path string `json:"path,omitempty"`
 }