add SCRAM-SHA-256 postgres support and ruby 2.5.7 test

[fe80]

Hello,

Since pg 10 you can use the sha256 password and that the default value with pg14.

I use a go program I've found for create this code https://github.com/supercaracal/scram-sha-256/blob/master/main.go

I've add too the test spec test for 2.5.7 (last ruby version of puppet6)

[fe80]

Hello,

I've apply the same code on Puppet postgresql module https://github.com/puppetlabs/puppetlabs-postgresql/pull/1313

[duritong]

can you rebase? :-D