Support for dumping extended statistics

Hi Team,

In order to restore dumped extended statistics (stxdndistinct, stxddependencies, stxdmcv) we need to provide input functions to parse pg_distinct/pg_dependency/pg_mcv_list strings.

Today we get the ERROR "cannot accept a value of type pg_ndistinct/pg_dependencies/pg_mcv_list" when we try to do an insert of any type.

Approch tried:
- Using yacc grammar file (statistics_gram.y) to parse the input string to its internal format for the types pg_distinct and pg_dependencies
- We are just calling byteain() for serialized input text of type pg_mcv_list.

Currently the changes are working locally, I would like to push the commit changes to upstream if there any usecase for postgres. Would like to know if there any interest from postgres side.

Regards,
Hari Krishna