Serializing `Vector{Array}` flattens elements

```
julia> tab2 = (x = [rand(2,2) for _ in 1:10], );

julia> io2 = IOBuffer()
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1)

julia> Arrow.write(io2, tab2)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=736, maxsize=Inf, ptr=737, mark=-1)

julia> seekstart(io2)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=736, maxsize=Inf, ptr=1, mark=-1)

julia> tab2_in = Arrow.Table(io2)
Arrow.Table: (x = [[0.0854794818573692, 0.6156692467610922, 0.4816953048461168, 0.10732732583276494], [0.2699150559983352, 0.7190134247010207, 0.4620847068434688, 0.8086969500721466], [0.32599895190448236, 0.5913226835708243, 0.914412039189433, 0.04825070891100114], [0.19800356830304522, 0.30969840980302754, 0.7586621051179192, 0.8383737966971678], [0.46353328096692525, 0.7946259468169095, 0.9205855777866754, 0.30143398878132244], [0.7980760486814706, 0.3204359773634282, 0.5253557751574975, 0.8140218988518082], [0.9662048857122691, 0.17369565835410516, 0.18328845062159216, 0.5595773503180002], [0.5423562251958653, 0.6285041382249899, 0.31820822802763304, 0.42185178416765723], [0.6985239393699472, 0.8842247274247905, 0.5466899110422125, 0.2058087506887174], [0.352616487112392, 0.6636744200136235, 0.9140196029380305, 0.2594771305442407]],)

julia> tab2_in.x
10-element Arrow.List{Vector{Float64}, Int32, Arrow.Primitive{Float64, Vector{Float64}}}:
[0.0854794818573692, 0.6156692467610922, 0.4816953048461168, 0.10732732583276494]
[0.2699150559983352, 0.7190134247010207, 0.4620847068434688, 0.8086969500721466]
[0.32599895190448236, 0.5913226835708243, 0.914412039189433, 0.04825070891100114]
[0.19800356830304522, 0.30969840980302754, 0.7586621051179192, 0.8383737966971678]
[0.46353328096692525, 0.7946259468169095, 0.9205855777866754, 0.30143398878132244]
[0.7980760486814706, 0.3204359773634282, 0.5253557751574975, 0.8140218988518082]
[0.9662048857122691, 0.17369565835410516, 0.18328845062159216, 0.5595773503180002]
[0.5423562251958653, 0.6285041382249899, 0.31820822802763304, 0.42185178416765723]
[0.6985239393699472, 0.8842247274247905, 0.5466899110422125, 0.2058087506887174]
[0.352616487112392, 0.6636744200136235, 0.9140196029380305, 0.2594771305442407]
```

At first I thought this was just some cleverness with how Arrow was serializing a "column matrix" (e.g. a n-by-1 matrix). I guess it's probably not possible to store the metadata about the size of the elements if each element can potentially have its own size? Maybe using StaticArrays is called for?

Start by reproducing the Julia example with Arrow.write and Arrow.Table for a Vector{Array}. Investigate how the round trip represents each matrix element, and consider the issue complete when nested arrays retain their individual dimensions instead of being flattened.

Written by the indexing model from the issue text.