Add support for Slack conversations.open method

[ksn135]

Description

Suppose you have slackIDs of your users in your database and you need to send a message to several of them BUT simultaneously to one channel (no matter if it is created or not), instead of sending a direct message to each of them separately.
We can implement a special conversations method that will return the name of an existing channel using the Slack API or create it automatically with the set of users passed as a parameter to this method.
Inside the SlackTransport::doSend method, if this option was set earlier, an additional request to the Slack API will be executed in the response, which will contain the name of the target channel, which will be stored in the array of options by the recipient_id key.

API Details: https://api.slack.com/methods/conversations.open

Request:

curl --location 'https://slack.com/api/conversations.open' \ 
   --header 'Content-Type: application/json; charset=utf-8' \
   --header 'Authorization: Bearer xoxb-XXXXXXXXXXXXX-YYYYYYYYYYYYY-ZZZZZZZZZZZZZZZZZZZZZZZZ' \
   --data '{
      "users": "U0XXXXXXXX1,U0XXXXXXXX2",
   }'

Response:

{"ok":true,"no_op":true,"already_open":true,"channel":{"id":"D0XXXXXXXX1"}}

Example

use Symfony\Component\Notifier\Bridge\Slack\SlackOptions;
use Symfony\Component\Notifier\Bridge\Slack\SlackTransport;
use Symfony\Component\Notifier\Message\ChatMessage;
use Symfony\Component\Notifier\Message\MessageInterface;

$chatMessage = new ChatMessage('New Symfony Feature');

$options = (new SlackOptions())
// use string parameter directly in conversation.open API call
    ->conversations('U0XXXXXXXX1,U0XXXXXXXX2') 
// OR allow to specify array
// !!! Implementor MUST perform array_filter and array_unique calls !!!
    ->conversations(['U0XXXXXXXX1', 'U0XXXXXXXX2', null, false, 'U0XXXXXXXX1'])
// OR even allow to specify recipients dynamically
    ->conversations(function(
        SlackTransport $transport,
        MessageInterface $message
    ): array {
        return ['U0XXXXXXXX1', 'U0XXXXXXXX2'];
    })
;
$chatMessage->options($options);

$chatter->send($chatMessage);