GENESIS_BLOCK_HASH constants

[DOBEN]

Task description

Add hardcoded constants TESTNET_GENESIS_BLOCK_HASH and MAINNET_GENESIS_BLOCK_HASH to the SDKs. These constants are frequently used by the dapps. STAGENET_GENESIS_BLOCK_HASH won't be added for flexibility since the network might be restarted if needed.
https://github.com/Concordium/concordium-dapp-examples/blob/e34d1a55e92bb912695147f9561afe4af6af2620/compliant-reward-distribution/indexer-and-server/src/constants.rs#L5

The variable should be in bytes but their display trait has a hexDecimal representation.
pub const TESTNET_GENESIS_BLOCK_HASH: [u8; 32] = [
66, 33, 51, 45, 52, 225, 105, 65, 104, 194, 160, 192, 179, 253, 15, 39, 56, 9, 97, 44, 177, 61,
0, 13, 92, 46, 0, 232, 95, 80, 247, 150,
];

The current suggestion would be to add the constants to contracts-common in base (https://github.com/Concordium/concordium-base/tree/main/smart-contracts/contracts-common) (if it doesn't increase the module size of smart contracts without the constant - investigation needed) and make the constants available in smart contracts as well. (https://github.com/Concordium/concordium-rust-smart-contracts/blob/main/examples/cis5-smart-contract-wallet/src/lib.rs#L49).