Added Homomorphic addition/subtraction of ElgamalCiphertexts' example

[harsh4786]

This program example demonstrates on-chain homomorphic addition/subtraction of Elgamal ciphertexts generated on the client side by the user. This is added to the tests folder

[harsh4786]

@armaniferrante could you review this? It's a test example for adding elgamal ciphertexts on-chain, used the functionality from solana-zk-token-sdk

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