docs: added spec for 'incomingPaymentCreated' and schema for 'amount' in webhooks.yaml

[oana-lolea]

Updated the webhooks.yaml for OpenAPI by adding spec for incomingPaymentCreated event and for amount schema. Removed postinstall and copy-op-schemas from backend scripts.

Changes proposed in this pull request

• Added spec for incomingPaymentCreated event.
• Added spec for amount schema.
• Removed postinstall and copy-op-schemas scripts from backend.

Context

Added missing spec for incomingPaymentCreated event and amount schema in OpenAPI specs file, webhooks.yaml. This schema was already defined in the Open Payments resource server specs and copied using postinstall and copy-op-schemas scripts. These scripts were removed from package.json as they are no longer necessary.
This issue fixes #2920

Checklist

• Related issues linked using fixes #number
• Tests added/updated
• Make sure that all checks pass
• Bruno collection updated (if necessary)
• Documentation issue created with user-docs label (if necessary)
• OpenAPI specs updated (if necessary)

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[mkurapov]

Looks good, just one simplification