[R] convert Array of struct type to data frame columns

[asfimport]

Reporter: Romain Francois / @romainfrancois

PRs and other links:

• GitHub Pull Request #4593

_{Note: This issue was originally created as ARROW-5625. Please see the migration documentation for further details.}

[asfimport]

Romain Francois / @romainfrancois:
Issue resolved by pull request 4593
#4593