API: Add `rng.spawn()`, `bit_gen.spawn()`, and `bit_gen.seed_seq`

This makes the seed sequence interface more public facing by:
1. Adding `BitGenerator.seed_seq` to give clear access to `_seed_seq`
2. Add `spawn()` to both the generator and the bit generator as
   convenience methods for spawning new instances.

I somewhat remember that we always meant to consider making this
more public and adding such convenient methods, but did not do
so originally.
So, now, I do wonder whether it is time to make this fully public?

It would be nice to follow up at some point with a bit of best practices.
This also doesn't add it to the `RandomState`, although doing it via
`RandomState._bit_generator` is of course valid.
Can we define as this kind of access as stable enough that downstream
libraries could use it?  I fear that backcompat with `RandomState`
might make adopting newer things like spawning hard for libraries?

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