PostgreSQL: Adds 'FullyQualifiedDomainName' configuration member.

[ninjarobot]

This PR closes #881

The changes in this PR are as follows:

• PostgreSQL: Adds FullyQualifiedDomainName configuration member.

I have read the contributing guidelines and have completed the following:

• [x] Tested my code end-to-end against a live Azure subscription.
• [x] Updated the documentation in the docs folder for the affected changes.
• [x] Written unit tests against the modified code that I have made.
• [x] Updated the release notes with a new entry for this PR.
• [x] Checked the coding standards outlined in the contributions guide and ensured my code adheres to them.

If I haven't completed any of the tasks above, I include the reasons why here:

Below is a minimal example configuration that includes the new features, which can be used to deploy to Azure:

open Farmer
open Farmer.Builders
open Farmer.PostgreSQL

let myPostgres = postgreSQL {
    admin_username "adminallthethings"
    name "aserverformultitudes42"
    capacity 4<VCores>
    storage_size 50<Gb>
    tier GeneralPurpose
    add_database "my_db"
    enable_azure_firewall
}

let template = arm {
    add_resource myPostgres
    // The FQDN is the endpoint for accessing the server and will be populated in an output variable.
    output "fqdn" myPostgres.FullyQualifiedDomainName
}