Add Snake activation function

[zaleslaw]

The Snake activation function is missed in KotlinDL.

Read more about Snake

The desired PR addressing this issue should include:

• Implementation of activation class named as SnakeActivation (you can take inspiration from the implementation of HardSigmoid as reference) added to the Activations.kt file
• Documentation of activation function
• JUnit tests in api module
• Support for export of activation function to JSON (see ModelSaver.kt)
• Support for import of activation function from JSON (see ModelLoader.kt)

P.S. If you want to take this ticket, please leave the comment below
P.P.S Read the Contributing Guidelines.

The reference implementation could be taken from tensorflow-addons

[cagriyildirimR]

I would like to work on this issue.

[zaleslaw]

Will wait for your PR @cagriyildirimR !