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ARROW-16648: [GLib] Add MemoryPool wrapper (#13224)

This adds the MemoryPool class to the GLib bindings. It only adds some methods used when troubleshooting ARROW-14790, most importantly getting the allocated bytes of the default memory pool.

It is difficult to write meaningful tests with only these methods exposed.

Lead-authored-by: Sten Larsson <sten@burtcorp.com>
Co-authored-by: Sten Larsson <sten.larsson@gmail.com>
Co-authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>