[log] add otLogGenerateNextHexDumpLine() to generate hex dump (#9555)

This commit adds a public OT API to generate hex dump output line by
line. This function is then used for both `LogDump{}()` and frame
capture output by CLI `promiscuous` command (removing repeated
similar code) and harmonize the hex dump outputs.

tests/scripts/expect/cli-promiscuous.exp

@@ -76,9 +76,8 @@ expect "16 bytes from $addr: icmp_seq=1"
 expect_line "Done"
 
 switch_node 3
-expect -re {============================================\[len = +\d+]============================}
-expect -re {\|( ([0-9A-Z]{2}|\.\.)){16}\|( .){16}\|}
-expect -- "-----------------------------------------------------------------------------------"
+expect -re {==============================\[RX len=\d+\]==============================}
+expect -- "------------------------------------------------------------------------"
 send "promiscuous disable\n"
 expect_line "Done"