[C++][R][Python] Update ExecPlan bindings

[asfimport]

In ARROW-15061 the ability to add metadata to an ExecPlan was added. This should be exposed in the R and Python bindings. For the Python bindings this is blocked on ARROW-14292. Also linking ARROW-15515 to make sure we capture this addition in those docs.

Reporter: Matthijs Brobbel / @mbrobbel

PRs and other links:

• GitHub Pull Request #12694

_{Note: This issue was originally created as ARROW-15528. Please see the migration documentation for further details.}

[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 per project policy. 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.