Add softplus implementation in scipy.special

Hello Everyone,
It might be nice to have a numerically stable softplus implementation, ie np.log1p(np.exp(x))

This implementation can be based on the following stackoverflow answers :
https://cs.stackexchange.com/questions/110798/numerically-stable-log1pexp-calculation
https://stackoverflow.com/questions/44230635/avoid-overflow-with-softplus-function-in-python

It can have a good place in the scipy module as it has other applications apart from ML/AI like it's a quite natural penalty function in optimization if one desires a smooth penalty in some optimization problems.

All opinions are welcome. Let's discuss this?
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