Shared benchmark format for command line and CSV outputs

1. add `print_output_table_header_row` func to print the table header for benchmark output
2. modify the following benchmarks to include the table header
    - bench_ecdh.c
    - bench_ecmult.c
    - bench_internal.c
    - bench_recover.c
    - bench_schnorrsig.c
    - bench_sign.c
    - bench_verify.c

src/bench_ecmult.c

@@ -333,6 +333,7 @@ int main(int argc, char **argv) {
     secp256k1_ge_set_all_gej_var(data.pubkeys, data.pubkeys_gej, POINTS);
 
 
+    print_output_table_header_row();
     /* Initialize offset1 and offset2 */
     hash_into_offset(&data, 0);
     run_ecmult_bench(&data, iters);