Protect proxy password by not storing it between sessions

[JacobTrossing]

Fixes #8055 by storing the proxy password only during run time and not in the system preferences. Also adds a pop-up at the start of the program if the user has checked "Use proxy" and "Use authentication" to allow them to enter the password for the proxy. An image of this pop-up can be seen below.

We considered using encryption to add extra security but were unsure if it was needed. What are your opinions on this?

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Summarizing the requirements of the issue:

1. Ask for proxy password at first connect
2. Ask the user whether they want to store the password permanently
3. If no: The password is stored in-memory only
4. If yes: Store the password using the Windows credential manager (or other credential managers). See Do not save password in Preferences #8055 (comment) for a discussion on the libraries.

This PR addresses the "If no" case only. It does not address all the other requirements.

Could you comment on your success investigating the libraries listed at #8055 (comment) - to handle requirements 2 and 4?

I am not sure whether requirement 1 is easy to implement.