Implement C Data integration

[quinnj]

This starts work towards supporting teh C data interface for the arrow
format, as documented
here.

Currently in this PR, it includes struct definitions and basic
methods to allow getting a pointer to an ArrowSchema/ArrowArray
C-compatible struct that can then be populated by another
implementation. For example, with this PR, you can do:

using Arrow, PyCall
pd = pyimport("pandas")
pa = pyimport("pyarrow")
df = pd.DataFrame(py"""{'a': [1, 2, 3, 4, 5], 'b': ['a', 'b', 'c', 'd', 'e']}"""o)
rb = pa.record_batch(df)
sch = Arrow.CData.getschema() do ptr
    rb.schema._export_to_c(Int(ptr))
end
arr = Arrow.CData.getarray() do ptr
    rb._export_to_c(Int(ptr))
end

Currently, these ArrowSchema/ArrowArray structs are pretty bare
bones, but it at least lays some ground work for integration. Things we
still need/want to make all this nicer to use/work with:

• Type format string parsing/converting: we need to parse the type
  format strings as outlined
  here
  to figure out what type of data we'll get in the arrays. It'd
  probably be best to add a type field to the ArrowSchema struct that
  we'd populate when converting from CArrowSchema -> ArrowSchema
• Add a method like Arrow.ArrowVector(::ArrowSchema, ::ArrowArray)
  that produced a concrete ArrowVector subtype, like
  Arrow.Primitive, Arrow.List, etc. This will be a bit tricky,
  because have to follow all the same columnar layout trickery that we
  currently handle for IPC in the table.jl build methods. Perhaps we
  can refactor all that so we can re-use some code? Otherwise, we might
  just need to reimplement a bunch of that logic specific to converting
  ArrrowArrays.
• That should give a robust consuming story; for producing, we
  probably need a definition like
  Arrow.ArrowSchema(a::Arrow.ArrowVector) that produced a valid
  ArrowSchema, and then overloads per ArrowVector subtype like
  Arrow.ArrowArray(x::Arrow.Primitive) that produced the right
  ArrowArray for a concrete arrow array
• Then the last piece we need is just figuring out the right mechanics
  for providing a pointer to the CArrowSchema, CArrowArray structs
  once they're populated

If anyone would like to help out, I'm happy to provide as much guidance
as possible so others can get their feet wet in some arrow spec
nitty-gritty.

[quinnj]

cc: @sa-, @Moelf

[codecov]

Codecov Report

Merging #178 (005c946) into main (bdd0e54) will decrease coverage by 2.19%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
- Coverage   81.34%   79.15%   -2.20%     
==========================================
  Files          25       26       +1     
  Lines        3034     3118      +84     
==========================================
  Hits         2468     2468              
- Misses        566      650      +84     

Impacted Files	Coverage Δ
src/Arrow.jl	54.54% <ø> (ø)
src/cinterface.jl	0.00% <0.00%> (ø)

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