Allow a point type to be passed to `rand(M)`

[kellertuer]

Currently we can do

• rand(M) to generate a random point on M
• rand(M; vector_at = p) for a corresponding tangent vector that fits the type of p

This does not yet allow to – for example – generate a random PoincareBallPoint, unless one already has memory and uses the in-place variant.

So if we just have a point type P it would be neat to have a rand that generates a point of that type. We probably might need a nothing or default_point_type(M) fallback or so. But it might be nice to have e.g. a

rand(M; point_type=P)

variant. Specifying both a point type and vector_at=would lead to an error probably? Something like that. It might be very nice to be used within JuliaManifolds/Manopt.jl#533.