Add option to eagerly connect to the Tor network

[riastradh-brave]

With #14668 fixing #14617, there is now a small delay the first time you open a private tab with Tor -- longer if it's the first time you've used private tabs with Tor at all. We did this so that corporate users who are not using Tor do not get a stern talking to from the company security team, as some did when we connected to the Tor network as soon as Brave starts in 0.23.19. But users who do use Tor may want to reduce the delay. Some ideas for how to enable Tor users to reduce the delay:

• We could create an option to eagerly start Tor (spelled in some more obvious way). Users who check this option will get on the Tor network sooner after launching Brave. Once on the Tor network, there is no difference.
• We could automatically enable that option the first, or perhaps second, time you open a private tab with Tor, or if you actually navigate to Tor once or twice, or some heuristic that indicates you're actually using Tor.
• We could discard the idea altogether because more options means more complexity and the delay in Tor startup is not that bad.

[riastradh-brave]

Inferring intent to use Tor based on past activity is not a good look. If this is an option, it should be something that should be explicitly enabled, not inferred.

[tildelowengrimm]

If we use Tor for background tasks, we'll probably eagerly load it rather than lazy-loading it. So this issue probably becomes a part of brave/brave-browser#804

[We still shouldn't start Tor — or anything else — right away as part of start up. We should always wait a little while so that startup can stay fast.]