Added | string

[RashmitTopG]

What changes are being made and why?

Added a new StringFilter to the Kestra project, which converts numeric inputs and string representations of numbers into their string form. This feature enhances template flexibility by allowing users to handle numeric values as strings easily.
This PR closes #5528 .

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How the changes have been QAed?

Example flow code demonstrating the use of the string filter in a Pebble template

template: |

  {{ 123 | string }}   # Expected output: "123"
  {{ "456.78" | string }}  # Expected output: "456.78"
  {{ "invalid" | string }}  # Expected to raise a PebbleException

This example shows how the new StringFilter behaves with different input types.

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[RashmitTopG]

@anna-geller I've submitted a pull request adding a StringFilter to the Kestra project that converts numbers to strings. Please review and suggest improvements if any .