fix: write binary as hex strings in mysql text protocol

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Summary

Currently the text protocol write binary value directly into the packet, thus the client would get the original binary values and display it like this:

But actually the mysql client display the binary value as hex strings (MySQL (client v8.4.0, server v9.0.1)):

Through the client source code it only parse binary values as hex strings when using binary charset:

// See https://github.com/mysql/mysql-server/blob/596f0d238489a9cf9f43ce1ff905984f58d227b6/client/mysql.cc
// L3916-L3918 and L4033-L4034
tatic bool is_binary_field(MYSQL_FIELD *field) {
  return (
      (field->charsetnr == 63) &&
// ...
// when write results
if (opt_binhex && is_binary_field(field))
    print_as_hex(PAGER, cur[off], lengths[off], field_max_length);

So I wonder if it's proper to directly write hex strings when using text protocol :)

[CookiePieWw]

After further investigations, it seems MySQL server sends original binary value as well, while the charset of text protocol is actually binary one:

// See https://github.com/mysql/mysql-server/blob/f7680e98b6bbe3500399fbad465d08a6b75d7a5c/sql/field.cc
// This is an old version (v5.7), I guess there's no change in the newer versions here.
bool Field::send_text(Protocol *protocol)
{
  if (is_null())
    return protocol->store_null();
  char buff[MAX_FIELD_WIDTH];
  // Charset is set to `my_charset_bin` all the time no matter what protocol is.
  String str(buff, sizeof(buff), &my_charset_bin);
  String *res= val_str(&str);
  return res ? protocol->store(res) : protocol->store_null();
}

String *Field_blob::val_str(String *val_buffer MY_ATTRIBUTE((unused)),
			    String *val_ptr)
{
  char *blob;
  // Directly copy the binary value, no conversion to hex strings.
  memcpy(&blob, ptr+packlength, sizeof(char*));
  if (!blob)
    val_ptr->set("",0,charset());	// A bit safer than ->length(0)
  else
    val_ptr->set((const char*) blob,get_length(ptr),charset());
  return val_ptr;
}

Then the binary to hex string conversion is still performed at client side. So maybe the correct fix is set the charset to binary here?
https://github.com/datafuselabs/opensrv/blob/6cbb80670116e8acce5dd4c473356e8df039804f/mysql/src/writers.rs#L134