3rd-party: Supports optional term-of-use for 3rd-party repos

[lebensterben]

When adding a 3rd-party repo, in addition to asking the user to whether to accept the certificate, also asks the user whether to accept the term of use of that repo, if it's provided.

• This could be done by allowing 3rd-party repos to put a file, for example TERMOFUSE, at https://<3rd-party-repo-url>/TERMOFUSE.
• Optionally, the first time when a user add a 3rd-party repo, we could also asks the user whether to accept that 3rd-party repos are not affiliated with Clear Linux, and should be responsible for various risks themselves.