Blog Postgres extension directories and path RFC

Based on pgsql-hackers discussions plus some further thinking through of
issues and questions. Cites packaging, Docker, and Postgres.app use
cases, and includes proposals to:

*   Install extensions into a single directory per extension
*   Add `pg_config` values for separate directories in which to install
    core, vendor, and site extensions
*   Add a search path for Postgres to find extensions at run time
*   Modify PGXS to install extensions in the new locations and with the
    proposed directory structures
*   Deprecate the `directory` and `module_pathname` control file variables
*   Add extension preload GUCs to complement the shared preload library
    GUCs
*   In the future: Deprecate `LOAD` and the shard preload library GUCs