[Filter/Custom] Support flexible dimension

[Filter/Custom] Support flexible dimension

- Allow to configure input/output dimension in run-time.
- After pad-cap negotiation, if gstremaer somehow fixes input-dimenson,
 a filter subplugin can configure output-dimension accordingly.
- An example custom filter, "passthrough_variable", supports any
 arbitrary tensor for passthrough operations.
- Filter/Main and Filter/Custom has been amended during the development
 of the example custom plugin.

FOUND BUG (mentioned as "known bug" in the code):
- If GST-BaseTransform tries to fixate pad-caps without getting
 any hints on the input dimension, fixate vmethod fixes the input
 dimension to 1:1:1:1. We need to implement non-fixed pad-cap
 declarations in fixated vmethod.
- The disabled test case #4 in tests/nnstreamer_filter_custom
 is the corresponding regression detector.
- However, this bug is to be fixed with another commit.

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