This commit adds function postgres_fdw_get_connections() to return
the foreign server names of all the open connections that postgres_fdw
established from the local session to the foreign servers. This function
also returns whether each connection is valid or not.

This function is useful when checking all the open foreign server connections.
If we found some connection to drop, from the result of function, probably
we can explicitly close them by the function that upcoming commit will add.

This commit bumps the version of postgres_fdw to 1.1 since it adds
new function.

Author: Bharath Rupireddy, tweaked by Fujii Masao
Reviewed-by: Zhijie Hou, Alexey Kondratov, Zhihong Yu, Fujii Masao
Discussion: https://postgr.es/m/2d5cb0b3-a6e8-9bbb-953f-879f47128faa@oss.nttdata.com