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().