curl: add write-out variable 'tls_earlydata'

Add the write-out variable 'tls_earlydata' to inspect the amount of
TLSv1.3 early data that had been sent.

Closes #15956

src/tool_writeout.h

@@ -97,6 +97,7 @@ typedef enum {
   VAR_STARTTRANSFER_TIME,
   VAR_STDERR,
   VAR_STDOUT,
+  VAR_TLS_EARLYDATA_SENT,
   VAR_TOTAL_TIME,
   VAR_URLNUM,
   VAR_NUM_OF_VARS /* must be the last */