Feature: add user-defined index access method

[hw118118]

The purpose of the commit is in support of user-defined index access method. Currently, there are many restrictions for index access methods and new index access methods are incompatible with internal framework. At the same time, it's impossible for user to change default index access method. So I build some hooks that are for changing default behavior and being compatible for those strange restrictions.

For express the meaning of the modification, I show a example as below.

For example , I want to add 7 kinds of new index access method that based on diffrent storage engine (maybe unionstore storage) and they are correspond to internal index access methods as below:

• usbtree ---btree
• usgin --- gin
• usgist --- gist
• usspgist --- spgist
• ushash --- hash
• usbitmap --- bitmap

The main diffrence between them is index data can separate from other data and support Compute and Storage Separation.

Suppose we want to use these new functions, we can create a new extension and we can run sql （for example by create extension unionstore;) command and you can find new access methods :

regression=# select * from pg_am;
  oid   |    amname     |         amhandler         | amtype
--------+---------------+---------------------------+--------
      2 | heap          | heap_tableam_handler      | t
    403 | btree         | bthandler                 | i
    405 | hash          | hashhandler               | i
    783 | gist          | gisthandler               | i
   2742 | gin           | ginhandler                | i
   4000 | spgist        | spghandler                | i
   3580 | brin          | brinhandler               | i
   7024 | ao_row        | ao_row_tableam_handler    | t
   7166 | ao_column     | ao_column_tableam_handler | t
   7013 | bitmap        | bmhandler                 | i
  16394 | union_store   | heap_tableam_handler      | t
  16402 | ushash        | ushashhandler             | i
  16403 | usbtree       | usbthandler               | i
  16404 | usgist        | usgisthandler             | i
  16405 | usgin         | usginhandler              | i
  16406 | usspgist      | usspghandler              | i
  16407 | usbrin        | usbrinhandler             | i
  16408 | usbitmap      | usbmhandler               | i
  22820 | heap2         | heap_tableam_handler      | t
 111831 | ao_row_testam | ao_row_tableam_handler    | t
 111847 | ao_col_testam | ao_column_tableam_handler | t
 111850 | heap_testam   | heap_tableam_handler      | t

So you can use these new index access methods as internal index access method. At the same time, I add new guc variable and you can correct default index type by set default_index_type = usbtree or show relevant info by show default_index_type;

In conclusion, it's more flexible and it decoupe from other part.