feat: generateLicenseToken for private key assertion

[tomasstark]

This PR adds a new method generateLicenseToken to allow for generating a token using private key JWT assertion. This is to reach feature parity with TS SDK.

⚠️ This is in draft mode because Supertab Connect currently support client credentials flow only. ⚠️

[Copilot]

Pull request overview

Adds support for generating a license token via private key JWT client assertion (to match TS SDK capabilities), exposing it as a new static API and wiring it through the customer token client, with accompanying docs and tests.

Changes:

• Introduces SupertabConnect::generateLicenseToken() (static) and LicenseTokenClient::generateLicenseToken() (instance).
• Refactors token exchange internals in LicenseTokenClient (shared content matching + shared POST/response parsing).
• Adds PHPUnit coverage for EC/RSA assertions, claims, request shape, caching, and error cases; updates README with usage.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/SupertabConnect.php	Adds new static entrypoint delegating to LicenseTokenClient.
src/Customer/LicenseTokenClient.php	Implements JWT assertion flow + refactors token request/response handling.
tests/SupertabConnectTest.php	Adds coverage for the new static method.
tests/Customer/LicenseTokenClientTest.php	Adds comprehensive tests for JWT assertion flow.
README.md	Documents the new method and provides an example.

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