feat: Added expire and issued timestamps in attachment

[harrrson]

Added methods to extract expiration and issued timestamps from url

dpp::embed test{};
test.set_title("Test for file")
    .add_field("filename", fileAttachment.filename)
    .add_field("description", fileAttachment.description)
    .add_field("url", fileAttachment.url)
    .add_field("proxy_url", fileAttachment.proxy_url)
    .add_field("Expiration timestamp", std::to_string(fileAttachment.get_expire_time()))
    .add_field("issuing timestamp", std::to_string(fileAttachment.get_issued_time()));

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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[braindigitalis]

approved and good to merge, would be nice if there were unit tests for this though

[Jaskowicz1]

approved and good to merge, would be nice if there were unit tests for this though

A todo list for stuff would be lovely