[ADD] sign_stamp: add custom Stamp field in Sign app

[rodh-odoo]

Implemented a new 'stamp' sign item that displays company logo and details (name, address, city, country, VAT). External users can upload a logo and edit details via a custom dialog. Stamp is saved and rendered properly during signing and in the final PDF. Includes fallback for missing logo and handles user input cleanly.

[robodoo]

[deso-odoo]

For commit msg what I prefer is ->if it is an improvement

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task-xyz

->in case of a bug fix

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task-xyz