Initiate or add to existing stake

Creates a new staking position and returns its unique ID. For Ethereum compounding validator (EIP-7251): when the 'id' of an existing compounding validator position is provided, adds to that position; otherwise creates a new position. For Ethereum legacy validator: creates a new position regardless of existing delegations. For Cosmos chains and Ethereum liquid staking (Lido): automatically add to existing positions for the same validator provider and same vault account if one exists, otherwise create a new position. For Solana and Polygon: always create new positions regardless of existing delegations.