Create maps recursively for "extra" metadata

[san650]

As part of fixing hexpm/hex#1092 we need to parse nested maps from the metadata.config file so these can be saved in the Package schema.

Related PR hexpm/hex#1096

If we don't, when trying to send nested maps as part of the Package's extra field in the mix.exs file it will raise this error in hexpm/hexpm API:

  1) test POST /api/packages/:name/releases create release and new package as user (HexpmWeb.API.ReleaseControllerTest)
     test/hexpm_web/controllers/api/release_controller_test.exs:30
     ** (Protocol.UndefinedError) protocol Jason.Encoder not implemented for type Tuple, Jason.Encoder protocol must always be explicitly implemented. This protocol is implemented for the following type(s): Any, Atom, BitString, Date, DateTime, Decimal, Ecto.Association.NotLoaded, Ecto.Schema.Metadata, Float, Hexpm.Accounts.RecoveryCode, Integer, Jason.Fragment, Jason.OrderedObject, List, Map, NaiveDateTime, Time, Version

     Got value:

         {"bar", [{"a", "b"}]}

     stacktrace:
       (jason 1.4.4) lib/jason.ex:213: Jason.encode_to_iodata!/2
       (postgrex 0.19.3) lib/postgrex/type_module.ex:1084: Postgrex.DefaultTypes.encode_params/3
       (postgrex 0.19.3) lib/postgrex/query.ex:75: DBConnection.Query.Postgrex.Query.encode/3
       (db_connection 2.7.0) lib/db_connection.ex:1449: DBConnection.encode/5
       (db_connection 2.7.0) lib/db_connection.ex:1549: DBConnection.run_prepare_execute/5
       (db_connection 2.7.0) lib/db_connection.ex:772: DBConnection.parsed_prepare_execute/5
       (db_connection 2.7.0) lib/db_connection.ex:764: DBConnection.prepare_execute/4
       (postgrex 0.19.3) lib/postgrex.ex:306: Postgrex.query/4
       (ecto_sql 3.12.1) lib/ecto/adapters/sql.ex:1150: Ecto.Adapters.SQL.struct/10
       (ecto 3.12.5) lib/ecto/repo/schema.ex:837: Ecto.Repo.Schema.apply/4
       (ecto 3.12.5) lib/ecto/repo/schema.ex:416: anonymous fn/15 in Ecto.Repo.Schema.do_insert/4
       (ecto 3.12.5) lib/ecto/multi.ex:897: Ecto.Multi.apply_operation/5
       (elixir 1.18.4) lib/enum.ex:2546: Enum."-reduce/3-lists^foldl/2-0-"/3
       (ecto 3.12.5) lib/ecto/multi.ex:870: anonymous fn/5 in Ecto.Multi.apply_operations/5
       (ecto_sql 3.12.1) lib/ecto/adapters/sql.ex:1400: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4
       (db_connection 2.7.0) lib/db_connection.ex:1756: DBConnection.run_transaction/4
       (ecto 3.12.5) lib/ecto/repo/transaction.ex:18: Ecto.Repo.Transaction.transaction/4
       (hexpm 0.0.1) lib/hexpm/repository/releases.ex:51: Hexpm.Repository.Releases.publish/8
       (hexpm 0.0.1) lib/hexpm_web/controllers/api/release_controller.ex:143: HexpmWeb.API.ReleaseController.handle_tarball/5
       (hexpm 0.0.1) lib/hexpm_web/controllers/api/release_controller.ex:1: HexpmWeb.API.ReleaseController.action/2

[ericmj]

Thank you! 💜