Expose `force_sample_times` in .infer()

[hyanwong]

Over in tskit-dev/tsdate#469 a user didn't realise about the force_sample_times option, probably because it's only in the match_samples function, and most beginners start by just calling tsinfer.infer().

I guess it would be trivial to add force_sample_times to the plain .infer() method too, which might make it more discoverable?