feat: add aggregate method support and improve error decoding

Add WithAggregate() option to use Multicall3's aggregate method instead
of aggregate3, which reverts the entire batch on any individual call
failure. Also refactor error decoding to use selector constants and
switch-based dispatch, and extract resolveError helper in batch
execution.

Initial commit: multicall Go library implementation

Add core multicall functionality with batch execution, generic helpers,
and Ethereum contract bindings for ERC20 and Multicall3.