add :default_vault configuration

[Gladear]

Hello 👋

It has been brought to my attention through our tooling that the classic schema when using
Cloak.Ecto creates compile-connected dependencies between the schema and the vault.
Indeed, using field :field, CustomType creates a compile dependency to CustomType,
which, in this library's use case, has in turn a runtime dependency to MyApp.Vault,
creating a compile-connected dependency.

I tried to think of some ways to avoid this:

• One of the solution was to use an API similar to :polymorphic_embed, and make
  Cloak.Ecto require to use its own field macro, but I couldn't really figure
  out how to make it work honestly 😶
• The second, much simpler solution, is to allow providing a
  config :cloak_ecto, default_vault: MyApp.Vault in the configuration, so that
  the CustomType module does not have to reference the MyApp.Vault module.
  That obviously does not cover the use-case where projects have multiple vaults...

I did a small PoC of the second option. I did not update the documentation, nor the
unit tests, as I'm just asking your opinion on this option.

Thanks in advance!

[a3kov]

In my opinion the vault should not be a genserver at all, but a module with it's own settings. See #62. Then automatically you won't have this issue