[C++] Pass task size / metrics in Executor API

[asfimport]

For now, our ThreadPool implementation would ignore those metrics, but other implementations may use it for custom ordering.

Example metrics:

• IO size (number of bytes)
• CPU cost (~ number of instructions)
• Priority (opaque integer? lower is more urgent)

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

PRs and other links:

• GitHub Pull Request #7225

_{Note: This issue was originally created as ARROW-8847. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @lidavidm @fsaintjacques Does this look ok?

[asfimport]

David Li / @lidavidm:
This looks good to me.

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
Issue resolved by pull request 7225
#7225