Function shard_of 

pub fn shard_of(address: &str, num_shards: u32) -> Result<u32, CoreError>

Determines the shard ID for a given bech32 address.

This implementation matches the Go reference: mx-chain-go/sharding/multiShardCoordinator.go

Arguments

• address - A bech32-encoded MultiversX address
• num_shards - The total number of shards in the network

Returns

The shard ID (0 to num_shards-1) or METACHAIN_SHARD_ID for metachain

Errors

Returns an error if the address is not a valid bech32 string