[C++][Python] Add a space after commas in pretty-print output

[asfimport]

Following ARROW-14798 we can have this kind of pretty-print output:

struct: [
  -- is_valid: all not null
  -- child 0 type: int64
  [0,1,2,3,4,...,45,46,47,48,49]
  -- child 1 type: list<item: string>
  [["crown","trunk","roots"],["roots","seeds"],...,[],["crown"]],
...,

IMHO it would read better as:

struct: [
  -- is_valid: all not null
  -- child 0 type: int64
  [0, 1, 2, 3, 4, ..., 45, 46, 47, 48, 49]
  -- child 1 type: list<item: string>
  [["crown", "trunk", "roots"], ["roots", "seeds"], ..., [], ["crown"]],
...,

Reporter: Antoine Pitrou / @pitrou

Related issues:

• PrettyPrint Improvements (is a child of)

PRs and other links:

• GitHub Pull Request #12420

_{Note: This issue was originally created as ARROW-15475. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @wjones127  

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
+1

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.

[kevingurney]

Note: I have opened #37981 which is related.