Download a json with signature

[arunge]

This is the first part of https://github.com/Pandora-IsoMemo/TraceR/issues/14#issue-2359921686

IMPORTANT: Signing a file will be possible only in the online version of the app!

In the online version during "Export with signature":

• the current userID (comming from the login handled via shinyproxy) must be saved and added to the .json file
• the content of the .json will be signed using openssl::rsa_keygen(), compare https://github.com/Pandora-IsoMemo/TraceR/pull/10/files
  • before hosting the app we will generate private and public keys that will only live on our shinyproxy server (and in a tool for storing passwords/credentials...)
  • when the app is running online, we can create a signature (openssl::signature_create()) using our private key (via an environment variable) for our file to be exported
  • export of the file containing the signature

@f-lukas as discussed, please

• [x] first test & merge the PR: https://github.com/Pandora-IsoMemo/TraceR/pull/19
• [x] add a new button to download a signed .json file (hidden when not running the online version) and feel free to open sub-issues as needed

[f-lukas]

I am closing here as the signed download is now available in the beta app.