Add Type information generation

[ProfFan]

We should have the ability to generate .pyi type stubs.

[varunagrawal]

I had a thought where we could use stubgen and then rsync to merge the .pyi files into the package.

[ProfFan]

Stubgen is not getting all the type information. gtsam-stubs is made with stubgen and (many) manual tweaks, but it's still incomplete. Also stubgen don't have documentation.

[varunagrawal]

Maybe we can get a simple version working which will at least allow autocomplete?

[ProfFan]

That is actually not difficult. I think it would look like the pybind generator.