MINOR: sample: Define new struct sample_storage

This struct is used to store a sample constant. The size of this
struct is less than the struct sample. This struct only contains
a constant and doesn't need the "ctx" nor the "flags".

include/types/sample.h

@@ -236,6 +236,18 @@ struct sample {
 	union smp_ctx ctx;
 };
 
+/* Used to store sample constant */
+struct sample_storage {
+	int type;                 /* SMP_T_* */
+	union {
+		unsigned int    uint;  /* used for unsigned 32bits integers and booleans */
+		int             sint;  /* used for signed 32bits integers */
+		struct in_addr  ipv4;  /* used for ipv4 addresses */
+		struct in6_addr ipv6;  /* used for ipv6 addresses */
+		struct chunk    str;   /* used for char strings or buffers */
+	} data;                        /* sample data */
+};
+
 /* Descriptor for a sample conversion */
 struct sample_conv {
 	const char *kw;                           /* configuration keyword  */