[4/n] Let user request a specific TTL for a token

[david-crespo]

Closes #8147.

Built on #8137, #8214, and #8227.

This is pretty straightforward, I think. The user gives us a TTL in seconds at token request time. We store it on the request row. When they come back in the later step to confirm the code and generate the token, we retrieve the TTL, validate that it is less than the silo max (if one is set), and we use it to generate the time_expires timestamp, which cannot be changed later.

One slightly surprising bit is that we can't validate the TTL against the silo max at initial request time because we don't have any idea what silo the user is associated with until the confirm step. So probably want to make sure we are handling TTL validation errors nicely in the web console, because I think that's where they will show up.