Implement `entropy` dispatches for distributions

Description

#7530 added a helper to compute the closed-form mean of a distribution, and #8084 adds variance.

One last useful helper that pops up in applications is the entropy. It's used in ML as a regularization term (e.g. in reinforment learning models that learn a policy function like DDPG), and can be used to pick priors (e.g. in pz.maxent.

It could be nice to have these around. I'm marking as request discussion because I realize by adding all these different moments and statistics we're going down a bit of a rabbit hole, and maybe someone wants to step in and say stop now.