[GLib] Add garrow_numeric_array_mean()

[asfimport]

Reporter: Yosuke Shiro / @shiro615
Assignee: Yosuke Shiro / @shiro615

PRs and other links:

• GitHub Pull Request #3889

_{Note: This issue was originally created as ARROW-4859. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 3889
#3889