Converters: support external subplugins

Converters: support external subplugins

Allow to add external converters in run-time as subplugins.

Policies to be determined:
- Build & integration policies for external converters.
- Would we allow a few "basic" external converters available by default? (without dlopen)
- Tizen feature keys and privileges along with "NOT SUPPORTED" handling.
- Which profiles would use which of external converters?

TODO:
- Implement multiple external subplugins for unit tests

Changes in V2:
- Typo fixed (comments)

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>