pg_hosts: Add pg_hosts_file_rules()

Hello,

While working on the configuration file format proposal, I noticed
that we are missing pg_hosts_file_rules, the pg_hosts version of
pg_hba_file_rules.

The attached patch adds this functionality, mirroring how the hba
version is implemented.

One detail I am definitely unsure about is the first rule_number
column, since pg_hosts is explicitly unordered, we might not want to
add it. I included it in 0001 for parity with hba_file_rules and
completeness.