Field Occurrence.breadcrumbs is broken for MySQL/MariaDB

[phihos]

Describe the bug

Hey folks, I recently upgraded from v0.3.0 to v0.6.0 and the detail page of any error crashed with

** (ArgumentError) cannot load `"[]"` as type {:array, :string} for field :breadcrumbs in %ErrorTracker.Occurrence{__meta__: #Ecto.Schema.Metadata<:loaded, "error_tracker_occurrences">, id: nil, reason: nil, context: nil, breadcrumbs: nil, stacktrace: nil, error_id: nil, error: #Ecto.Association.NotLoaded<association :error is not loaded>, inserted_at: nil}

I already debugged this and it seems that the Ecto adapter Ecto.Adapters.MyXQL correctly stores arrays (like "[]" for an empty array), but crashes on retrieval.
I think there are two hints that we can not make this work just by configuration:

1. I was not able to find documentation on how to make type :array work with MyXQL. The closest thing I found was this entry in the docs that describes how to store type :map in a JSON column. But no word about :array.
2. When looking at the MyXQL source we see no implementation for def loaders(:array, type)....

In order to fix this I propose implementing a custom type. I already implemented a proposal that I will show in a PR soon.

To Reproduce

Steps to reproduce the behavior:

1. Configure a MySQL/MariaDB db as storage backend.
2. Generate some error.
3. Try to navigate to the error detail page (like /errors/1).
4. Internal server error.

Expected behavior

Detail page should not crash.