Make it possible to supply custom headers along with StreamableHttp

[thekid]

Example:

use io\modelcontextprotocol\{McpClient, StreamableHttp};
use util\cmd\Console;

$client= new McpClient(new StreamableHttp($argv[1])->with(['Authorization' => 'Token '.$argv[2]]));

// Initialize
$init= $client->initialize();
Console::writeLine('=> Connected to ', $argv[1], ': ', $init);

// Fetch tool list
$response= $client->call('tools/list');
Console::writeLine($response->value());