Add optional fallback URL for critical networks

[dmytrotkk]

For critical infrastructure, we need an optional fallback for ABIs if skale-contracts fail to load contracts from Github.

The ideal solution should work without any Github interactions - if library can't download required ABI from Github for any reason, it should try to load a file from S3 bucket with name [project-name]-[chain-id]-[version].json or something similar.

Fallback ABI files will be uploaded only for critical networks (mainnet / testnet) - we can add a step to CI or just upload manually after mainnet contracts update.