Consider changing the GDK::Config DSL

I've had remarks about several people who find the current GDK::Config DSL quite confusing and the code behind it in GDK::ConfigSettings hard to understand.

Current state

Currently the DSL is used as something like:

    postgresql do |p|
      p.port { read!('postgresql_port') || 5432 }
      p.replication_user 'gitlab_replication'
      p.geo do |g|
        g.port { read!('postgresql_geo_port') || 5432 }
        g.dir { config.gdk_root.join('postgresql-geo') }
      end
    end

Proposal

The proposal is to move to something like:

config :postgresql do
  integer :port, { read!('postgresql_port') || 5432 }
  string :replication_user, 'gitlab_replication'
  config :geo do
    integer :port, { read!('postgresql_geo_port') || 5432 }
    path :dir, { config.gdk_root.join('postgresql-geo') }
  end
end

(Some inspiration was taken from https://dry-rb.org/gems/dry-configurable, /ht @nolith)

Each item will be a ConfigSetting instance, but depending on the method used to create it, it might have different type checking.

Advantages

• Less method_missing magic. Especially no more arity tricks
• Type validation built-in from the start: #659 (closed)
• Easier deprecation of loose files (see !888 (closed) and #658 (closed))

Alternative

If we do not like the different method names, we could also:

config :postgresql do
  config :port, type: :integer { read!('postgresql_port') || 5432 }
  config :replication_user, type: :string, { 'gitlab_replication' }
  config :geo do
    config :port, type: :integer, { read!('postgresql_geo_port') || 5432 }
    config :dir, type: :path, { config.gdk_root.join('postgresql-geo') }
  end
end