[Aggregator] initial commit

[Aggregator] initial commit

Plugin to aggregate tensor stream. (used gst-adapter)

1. add properties to set output frames
 - frames-in : number of frames in input buffer
 - frames-out : number of frames in output buffer
 - frames-flush : number of frames to flush data

2. testcase (stream with aggregater) will be added soon.

**Self evaluation:**
1. Build test: [*]Passed [ ]Failed [ ]Skipped
2. Run test: [*]Passed [ ]Failed [ ]Skipped

related issue : #474 [Advanced Queueing] Support aggregation

Signed-off-by: Jaeyun Jung <jy1210.jung@samsung.com>