isisd: Add SRv6 SID structure data structure

Add `struct isis_srv6_sid_structure` data structure to represent an SRv6
SID structure (as defined in RFC 8986 section #3.1).

The struct has the following members:
* loc_block_len: locator block length
* loc_node_len: locator node length
* func_len: function length
* arg_len: argument length

Signed-off-by: Carmine Scarpitta <carmine.scarpitta@uniroma2.it>

isisd/isis_srv6.h

@@ -18,6 +18,14 @@
 #define SRV6_MAX_H_ENCAPS 2
 #define SRV6_MAX_END_D 5
 
+/* SRv6 SID structure */
+struct isis_srv6_sid_structure {
+	uint8_t loc_block_len;
+	uint8_t loc_node_len;
+	uint8_t func_len;
+	uint8_t arg_len;
+};
+
 /* SRv6 SID not bound to any adjacency */
 struct isis_srv6_sid {
 	struct isis_srv6_sid *next;