[Enhancement] Update constructor client

[Cannonb4ll]

$hetznerClient = new \LKDev\HetznerCloud\HetznerAPIClient(
    $key,
    'https://api.hetzner.cloud/v1/',
    'ploi.io/1.0.0'
);

If the URL would ever change, I would have to manually change this in my source, because I have a custom agent passed in. Would be nice if there would be 2 additional methods:

Set URL:

$hetznerClient->setApiUrl('http://...')

Set User Agent:

$hetznerClient->setUserAgent('ploi.io/1.0.0')

Client instantiator:

$hetznerClient = new \LKDev\HetznerCloud\HetznerAPIClient($key);

Total picture:

$hetznerClient = new \LKDev\HetznerCloud\HetznerAPIClient($key);

$hetznerClient->setApiUrl('http://...');

$hetznerClient->setUserAgent('ploi.io/1.0.0')