Extend Table AM routine to get total blocks can be analyzed

Hi Hackers,

Table AM routine already provided two custom functions to fetch sample blocks and sample tuples,

however, the total blocks the ANALYZE can scan are still restricted to the number of physical blocks

in a table, this doesn't work well for storages which organize blocks in different ways than the heap.

Here is proposing to add a new method named scan_analyze_total_blocks() to provide more flexibility,

it can return physical or logical blocks number which depends on how the table AM implement

scan_analyze_next_block() and scan_analyze_next_tuple().