feat: create getAccountGroup function

[igorsenych-cw]

Main changes

A new getAccountGroup function has been added to the YieldStreamer contract.

    /**
     * @notice Returns the group ID for a given account
     * @param account The address of the account
     * @return The group ID
     */
    function getAccountGroup(address account) external view returns (bytes32) {
        return _groups[account];
    }

Test coverage

New functionality needs to be tested.