Add DatedSwapRateHelper

[eltoder]

Also, move common functionality into SwapRateHelperBase to avoid
duplication. Reuse it between vanilla and overnight swaps.

[eltoder]

@lballabio I'll rebase this on top of the final version of #1911, so please only look at the second commit.

I wanted to get your feedback on the approach -- do you think this is worth it, or should I just copy-paste some code from SwapRateHelper? One nice thing here is that we get custom pillars support for DatedOISRateHelper for free and also fix some inconsistencies in how it sets dates.

[eltoder]

I also noticed that I previously added forwardStart to DatedOISRateHelper by mistake (it has no effect). Do you think it's not too late to remove it?

[coveralls]

coverage: 72.454% (-0.02%) from 72.477%
when pulling c2662ad on eltoder:feature/dated-swap-helper
into 72012a7 on lballabio:master.

[github-actions]

This PR was automatically marked as stale because it has been open 60 days with no activity. Remove stale label or comment, or this will be closed in two weeks.

[lballabio]

Hi Eugene, apologies from dropping the ball on this one---do you think it's possible to extract some utility function instead of using inheritance for reuse? It might (or might not) be simpler to read...

[eltoder]

I'll take another look, but I doubt it. Most of the real work is already done by MakeVanillaSwap and MakeOIS. Rate helpers mostly do plumbing and use a lot of member variables, so factoring that into free functions is probably not going to help.

[lballabio]

Superseded by #2106.