feat(r): Add reticulate/Python integration

[paleolimbot]

This PR adds the s3 generics required to interact with Python objects via reticulate. Thie relies on nanoarrow for Python, which also exposes primitives for interacting with capsules. In theory we wouldn't need nanoarrow for Python for the Python -> R case if we could create and consume PyCapsules but this is at least a good start!

library(nanoarrow)
library(reticulate)

pa <- import("pyarrow", convert = FALSE)

x <- pa$array(c(1, 2, 3))

(array <- as_nanoarrow_array(x))
#> <nanoarrow_array double[3]>
#>  $ length    : int 3
#>  $ null_count: int 0
#>  $ offset    : int 0
#>  $ buffers   :List of 2
#>   ..$ :<nanoarrow_buffer validity<bool>[null] ``
#>   ..$ :<nanoarrow_buffer data<double>[3][24 b]> `1 2 3`
#>  $ dictionary: NULL
#>  $ children  : list()
(py_array <- r_to_py(array))
#> nanoarrow.Array<double>[3]
#> 1.0
#> 2.0
#> 3.0
py_to_r(py_array)
#> <nanoarrow_array double[3]>
#>  $ length    : int 3
#>  $ null_count: int 0
#>  $ offset    : int 0
#>  $ buffers   :List of 2
#>   ..$ :<nanoarrow_buffer validity<bool>[null] ``
#>   ..$ :<nanoarrow_buffer data<double>[3][24 b]> `1 2 3`
#>  $ dictionary: NULL
#>  $ children  : list()

Closes #810.

[paleolimbot]

If there are no objections I'll merge later today!