feat: implement event webhook fields in dpp::application class

[Commandserver]

https://discord.com/developers/docs/resources/application#application-object-application-structure

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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