Support get_server_public_key option

[samitani]

What

This PR fixes below MySQL authentication error with SSL-disabled connection.

Mysql2::Error: Authentication plugin 'caching_sha2_password' reported error: 
Authentication requires secure connection.

Background

This error only happen in case of first authentication under cleartext TCP connection. caching_sha2_password authentication requires a secure connection at first authentication for each users. The first authentication means that the cache is not ready yet at server-side. The reason why secure connection is required is that MySQL server need to know user password to create cache. After the cache is created, MySQL server does not request secure connection anymore. Please refer to the MySQL manual to know details.

For each user account, the first client connection for the user after any of the following operations must use a secure connection (made using TCP using TLS credentials, a Unix socket file, or shared memory) or RSA key pair-based password exchange:
https://dev.mysql.com/doc/refman/8.0/en/caching-sha2-pluggable-authentication.html

libmysqlclient provides MYSQL_OPT_GET_SERVER_PUBLIC_KEY option which enables clients to create a requested secure connection automatically during authentication. This PR carries it to mysql2.

Sample code

require 'mysql2'

port     = 3306
hostname = 'hostname'
username = 'user'
password = 'password'

# connect with SSL to issue FLUSH PRIVILEGES
client = Mysql2::Client.new(:host => hostname, :port => port, :username => username, :password => password, :ssl_mode => :required)

# force clear cache
client.query("FLUSH PRIVILEGES")

print "# without get_server_public_key\n"
# this returns Authentication requires secure connection.
begin
  client = Mysql2::Client.new(:host => hostname, :port => port, :username => username, :password => password, :ssl_mode => :disabled)
rescue => e
  printf("connect ng %s\n", e)
end

print "# with get_server_public_key\n"
begin
  client = Mysql2::Client.new(:host => hostname, :port => port, :username => username, :password => password, :ssl_mode => :disabled, :get_server_public_key => true)
  print "connect ok\n"
rescue => e
  printf("connect ng %s\n", e)
end

$ ruby ../test2.rb
# without get_server_public_key
connect ng Authentication plugin 'caching_sha2_password' reported error: Authentication requires secure connection.
# with get_server_public_key
connect ok

[sodabrew]

The method name set_get_... seems extraneous? Can it be plain get_server_public_key? nm, this is a setter. Weird. Reading more.

[sodabrew]

It's not necessary to test for this constant into its own HAVE_ constant, because we can directly test for the constant at time of use (see suggested change)

[samitani]

@sodabrew Hello, Thank you for reviewing my PR.
MYSQL_OPT_GET_SERVER_PUBLIC_KEY is defined as enum. If my understand correctly, ifdef does not work with enum.
https://github.com/mysql/mysql-server/blob/8.0/include/mysql.h.pp#L411-L448

[sodabrew]

You’re absolutely right!

[sodabrew]

Suggested change

	#ifdef HAVE_CONST_MYSQL_OPT_GET_SERVER_PUBLIC_KEY
	#ifdef MYSQL_OPT_GET_SERVER_PUBLIC_KEY

Code is correct as is

[sodabrew]

Thank you for the PR!