[Variant] Improve ergonomic for creating VariantArray

[alamb]

If we do this:

impl<'m, 'v, V: Into<Variant<'m, 'v>> FromIterator<Option<V>> for VariantArray {

then we can create a variant array from anything that converts cleanly to variant (e.g. i64 values or &str)?

Originally posted by @scovich in #8625 (comment)