Based on Caffe's Exponential Layer.

resolves: #19475 #18088
merge with extra: opencv/opencv_extra#853

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable
  Patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake

force_builders=Custom,Custom Win,Custom Mac
build_image:Custom=ubuntu-openvino-2020.3.0:16.04
build_image:Custom Win=openvino-msvs2019-2021.2.0
build_image:Custom Mac=openvino-2021.2.0

test_modules:Custom=dnn,python2,python3,java
test_modules:Custom Win=dnn,python2,python3,java
test_modules:Custom Mac=dnn,python2,python3,java

buildworker:Custom=linux-1
# disabled due high memory usage: test_opencl:Custom=ON
test_opencl:Custom=OFF
test_bigdata:Custom=1
test_filter:Custom=*

Xbuildworker:Custom=linux-4,linux-6
Xbuild_image:Custom=ubuntu-cuda:18.04