Move the .ply file readers out of re_sdk_types and into re_importer

[emilk]

re_sdk_types currently contains .ply parsing for two archetypes:

• crates/store/re_sdk_types/src/archetypes/points3d_ext.rs
• crates/store/re_sdk_types/src/archetypes/gaussian_splats3d_ext.rs

Both are almost entirely about the file format rather than about the log type. Our log-type definitions shouldn't carry format-specific IO — that belongs in re_importer (or a reader crate it owns), which is where every other format lives.

A direct Archetype::from_ply_* constructor would only make sense for something like a GaussianSplats3DPly with an opaque blob component, which is not what these are.

Raised by @grtlr in https://github.com/rerun-io/reality/pull/2621#discussion_r3620919768; deferred out of that PR to keep the diff reviewable.

[Wumpf]

there's more of that coming in #12730

let's first land that and then do the move to not further slow down that PR (there's been lots delays on that already)