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_ecdh.c

@@ -52,6 +52,8 @@ int main(void) {
     /* create a context with no capabilities */
     data.ctx = secp256k1_context_create(SECP256K1_FLAGS_TYPE_CONTEXT);
 
+    print_output_table_header_row();
+
     run_benchmark("ecdh", bench_ecdh, bench_ecdh_setup, NULL, &data, 10, iters);
 
     secp256k1_context_destroy(data.ctx);