Option to update existing signature box (instead of always creating a new one)

[ridgey-dev]

Currently, the package automatically creates a new signature box with a random name for every signature action. This behavior leads to PDFs that contain both the newly created (signed) signature box and the original (unsigned) placeholder, which is not ideal for workflows where maintaining the integrity of pre-existing placeholders is critical.

To address this, I have created this pull request to enable the functionality of updating existing signature boxes instead of creating new ones. This enhancement ensures that the package can effectively handle documents with predefined signature placeholders without introducing redundancies.