New types and traits for (co-existing!) eager serialization

[teh-cmc]

This introduces SerializedComponentBatch, which will become the main type we use to carry user data around internally.

/// The serialized contents of a [`ComponentBatch`] with associated [`ComponentDescriptor`].
///
/// This is what gets logged into Rerun:
/// * See [`ComponentBatch`] to easily serialize component data.
/// * See [`AsComponents`] for logging serialized data.
///
/// [`AsComponents`]: [crate::AsComponents]
#[derive(Debug, Clone)]
pub struct SerializedComponentBatch {
    pub array: arrow::array::ArrayRef,

    // TODO(cmc): Maybe Cow<> this one if it grows bigger. Or intern descriptors altogether, most likely.
    pub descriptor: ComponentDescriptor,
}

The goal is to keep the ComponentBatch trait isolated at the edge, where it is used as a means of easily converting any data into arrow arrays, instead of simultaneously being used as a means of transporting data around through the internals.
ComponentBatch is here to stay, if only for its conversion capabilities.

This opens a lot of opportunities of improvements in terms of DX, UX and future features (e.g. generics).

The two code paths will co-exist for the foreseeable future, until all archetypes have been made eager.

• Part of Eager/early serialize of components to arrow in Rust and C++ #7245

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[emilk]

It's worth to also change the docstring to DEPRECATED - DO NOT USE or similar.

[emilk]

Still worth mentioning a few of the ways it can fail (e.g. overflowing an i32)