Create a progress bar for the plan validation

[RyuzakiKK]

Validating a plan could take a non trivial amount of time, depending on
the size of a plan and by the I/O speed.

With this commit we add a progress bar that shows the status of the
validation and prefixes the assembling progress bar with "Assembling ".

---

I added just the validation and not the actual "planning" because, from my testings, the planning is usually done nearly instantaneously and the progress percentage would just go from 0% to 100%.

[RyuzakiKK]

Example output when extracting an index file:

ValidatingPlan1    0.00%
ValidatingPlan1    4.43% 00m10s
ValidatingPlan1   10.65% 00m08s
ValidatingPlan1   18.52% 00m06s
ValidatingPlan1   23.07% 00m06s
ValidatingPlan1   31.06% 00m05s
ValidatingPlan1   35.51% 00m05s
ValidatingPlan1   42.22% 00m04s
ValidatingPlan1   44.63% 00m04s
ValidatingPlan1   50.05% 00m04s
ValidatingPlan1   53.87% 00m04s
ValidatingPlan1   59.34% 00m03s
ValidatingPlan1   66.79% 00m02s
ValidatingPlan1   70.55% 00m02s
ValidatingPlan1   88.07%
ValidatingPlan1  100.00% 7s
Assembling    0.00%
Assembling    5.22% 00m09s
Assembling   17.73% 00m04s
[...]

EDIT: In the previous commit I used "Validating", but given that this operation could be repeated if we have an invalid seed, I switched to "ValidatingPlan{number}" to include the attempt number.

[folbricht]

Looks good, thank you. If you want there's an opportunity to improve my old code here. If NewProgressBar() returned something like NullProgressBar that implements the ProgressBar interface instead of nil in the non-terminal case, you could get rid of all the nil checks for it thoughout the code (the if pb != nil bit)

[RyuzakiKK]

Sure, I should be able to do that tomorrow.

[RyuzakiKK]

With the latest revision I added the suggested NullProgressBar, and the progressbar for when we chunk an invalid seed.

[folbricht]

Thank you

[folbricht]

Hmm, looks like the MacOS build breaks due to a lib issue, likely need some built-tags.

[folbricht]

Nope, unrelated build issue in Go 1.18. Fixed in #218