Provide code for offline verification (Circom+Gnark)

[katiemckeon]

As a user
I want to see examples which decode and verify a proof returned by Sindri
so that I can easily use that data

Acceptance Criteria

Required to be satisfied in order to accept the issue. The example-code directory contains a subdir with code performing the following:

• Verify Circom/Groth16 proofs
• Verify Gnark/Groth16 proofs

[katiemckeon]

10/23: revised to exclude Halo2 offline verifier. Given the more complex nature of verifying Halo2 proofs, this part will be broken out into its own issue and tackled in a later sprint