Add integrator and child account support

[gogl92]

1️⃣ Is this something that is wanted/needed? Did you create an issue / discussion about it first?
Yes, this is needed to support PrintNode Integrator accounts. No prior issue/discussion was created.

2️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.
No, this PR contains a single, focused change to support PrintNode Integrator Child Accounts.

3️⃣ Does it include tests, if possible? (Not a deal-breaker, just a nice-to-have)
No, this PR does not include new tests.

4️⃣ Please include a thorough description of the improvement and reasons why it's useful.
This PR adds support for PrintNode Integrator Child Accounts by introducing a child_account request option. When provided, this option automatically adds the X-Child-Account header to all API requests, allowing integrators to scope operations to a specific child account. This is crucial for managing multiple client accounts under a single integrator account, as outlined in the PrintNode API documentation ("Account download & management – X-Child-Account").

Usage:
You can now specify a child_account ID in the request options:

$client->printers->all([], [
    'child_account' => 1234, // Child Account ID
]);

This option can also be supplied when instantiating the PrintNodeClient.

5️⃣ Thanks for contributing! 🙌

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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