[extra-files] Write a metadata file enumerating extra files
    
    Introduces a new metadata file to track arbitrary files added during the
    extra-files phase. This file is placed in the root of each tree and is
    called ``extra_files.json``. It is a JSON file containing a single
    object, which contains a "header" key with an object describing the
    metadata, and a "data" key, which is an array of objects, where each
    object represents a file. Each object contains the "file", "checksums",
    and "size" keys. "file" is the relative path from the tree root to the
    extra file. "checksums" is an object containing one or more checksums,
    where the key is the digest type and the value of that key is the hex
    digest. Finally, the size is the size of the file in bytes.
    
    For example:
    {
      "header": {"version": "1.0},
      "data": [
        {
          "file": "GPL",
          "checksums": {
            "sha256": "8177f97513213526df2cf6184d8ff986c675afb514d4e68a404010521b880643"
          },
          "size": 18092
        },
        {
          "file": "release-notes/notes.html",
          "checksums": {
            "sha256": "82b1ba8db522aadf101dca6404235fba179e559b95ea24ff39ee1e5d9a53bdcb"
          },
          "size": 1120
        }
      ]
    }
    
    Signed-off-by: Jeremy Cline <jeremy@jcline.org>