WIP: vagrant/base: add support for info.json inside boxes

[NeilHanlon]

n.b., I am very open to changing the interface if a different method is preferred, apologies in advance for not discussing before writing the implementation but my brain wanted to see it through :D

WIP, need to write some (more) tests.

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This change introduces a new <info> subelement to the <vagrantconfig> element which, if defined in a description, would populate the info.json file inside a vagrant box. This file is used by vagrant when displaying information about boxes on the system: https://developer.hashicorp.com/vagrant/docs/boxes/info

The content of this json file is free-form, so there are no specific keys which must be used. Thus, a configuration such as:

<vagrantconfig ...>
<info name="Author">My Name</info>
<info name="URL">https://example.com</info>
</vagrantconfig>

Would become an info.json file:

{
  "Author": "My Name",
  "URL": "https://example.com"
}

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