Create a framework agreement and signing link

Create a framework agreement and a signing link, for the given worker. Once the agreement is signed, the FrameworkAgreementSignedEvent webhook is executed.

This endpoint is idempotent for retries: if an unsigned, non-expired framework agreement already exists for the worker and the requested isPayrollTaxReduction matches the worker's stored setting, the existing agreement and its signing link are returned instead of creating a new one. If the requested isPayrollTaxReduction differs (the worker changes their setting), the pending agreement - including its Stiply signing request - is cancelled and a new agreement reflecting the new setting is generated. A new agreement is likewise created when there is no pending agreement (e.g. the previous one expired or was already signed); any obsolete unsigned agreement is then replaced.