chore (sync service): setup sentry

[kevin-dp]

This PR configures Electric to track errors with sentry.
To enable sentry for the single tenant Electric app you need to pass a SENTRY_DSN env var.

I've tested this with the cloud deploy and errors are tracked successfully in sentry.

TODO:

• add stack_id and request_id meta data to the recorded error events (*)

(*) The easy way is to configure sentry to add the stack_id and request_id meta data to the event context. That way it will show up in the error's details but it won't be searchable filterable on those fields! So a better way would be to add that meta data as tags. But not sure if that is configurable out of the box, might need to write a custom LoggerHandler.

Adding the meta data as tags is possible by extending all Logger.metadata calls (from @icehaunter's recent PR) with a sentry: [tags: %{stack_id: stack_id, shape_handle: shape_handle}] key. Cf. https://github.com/getsentry/sentry-elixir/blob/master/lib/sentry/logger_handler.ex#L205 for more info.

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