[vectorized] [block] add new method get_data_type to avoid unnecessary copy by the method get_data_type - #7600

Proposed changes

1. add new method get_data_type for class Block to reduce the copy cost of get_data_types
2. change function erase_useless_column()'s loop control variable i to type int for safety reason

Close related #issue (replace it with issue number if it exists).

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