Allow apps to hook in their own locator

[jeremy]

When multiple apps collaborate and reference each others' Global IDs, they'll use different means to locate the URI references.

For example, locate an Active Resource class:

GlobalID::Locator.use :foo do |gid|
  FooRemote.const_get(gid.model_name).find(gid.model_id)
end

GlobalID::Locator.locate 'gid://foo/Account/1234'
# => FooRemote::Account id=1234

Or locate the referenced model in a search index:

GlobalID::Locator.use :bar, BarLocator.new

class BarLocator
  def locate(gid)
    @search_client.search name: gid.model_name, id: gid.model_id
  end
end

GlobalID::Locator.locate 'gid://bar/Account/1234'
# => BarResult name=Account id=1234

[jeremy]

Implemented by @kaspth ❤️

[kaspth]

Nice! Thanks, @jeremy ❤️