Add global default field/association value configuration.

[mcclayton]

Previously we only supported being able to set default values at the field/association level. This PR enables setting a global default for all fields/associations that evaluate to nil.

Global Config Setting

Blueprinter.configure do |config|
  config.field_default = "N/A"
  config.association_default = {}
end

Field-level/Associaion-level Setting

class UserBlueprint < Blueprinter::Base
  identifier :uuid

  view :normal do
    field :first_name, default: "N/A"
    association :company, blueprint: CompanyBlueprint, default: {}
  end
end

The default value specified at the field/association level will override the value specified in the global config.

[AllPurposeName]

Rather than this ternary, I would prefer something like

Suggested change

	association_options.key?(:default) ? association_options.fetch(:default) : Blueprinter.configuration.association_default
	assosiation_options.fetch(:default, Blueprinter.configuration.association_default)

What do you think?

[mcclayton]

I think that makes sense to me, but I avoided this initially, because I literally wanted to do a check on whether the :default key was present in the options in case it was explicitly set to nil, we'd want to actually render nil instead of pass through to the global Blueprinter default.
Make sense? I don't know if that's an important enough behavior to preserve or not. Thoughts? 🤔

[AllPurposeName]

Yeah that totally makes sense. I could see someone switching it up like that if only one or two endpoints ignore the default and use nil instead.

[AllPurposeName]

Could we switch this example to be nil like the one we discussed above. Or alternatively add another test specifically for that functionality.

[mcclayton]

Yeah that's a great suggestion. I'll add another test case for that 👍

[philipqnguyen]

lgtm