[GLib] Rename *garrow_decimal128_array_get_value to *garrow_decimal128_array_format_value

[asfimport]

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

PRs and other links:

• GitHub Pull Request #2184

_{Note: This issue was originally created as ARROW-2749. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 2184
#2184