Allow rerun::Collection to borrow strided & offseted data

[Wumpf]

In order to support borrowing of existing image data well, we have to be able to support arbitrary strides on borrowed pointers.

To that end all our serialization methods have to work with rerun::Collection instead of getting a pointer and a size.

Talking to a user, not only strides would be useful but also byte offsets. Example: Struct like this:

struct PointAndColor { float x,y,z; uint32_t color; }
https://github.com/adujardin/zed-rerun-io/blob/dfaa374e900ff60720060759f7018e8dcdd9ae90/rerun_depth_sensing/src/main.cpp#L100

However, we likely can disregard owned data for this and focus solely on borrowed data.
Also, if it helps we can restrict this to POD data (data where we don't have to call a destructor)

Focusing on non-owned data allows to re-use the space taken by the deallocator pointer proposed in:

• Small size optimization & custom deallocator for rerun::Collection #4256