Dynamic schema prefix

[slashdotdash]

Support dynamic schema prefix for the projection versions table which can determine the schema based upon the event being projected.

This pull request extends support for the schema_prefix config option to support a one-arity function as a projector option:

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto,
    application: MyApp.Application,
    repo: MyApp.Projections.Repo,    
    name: "example_projection",
    schema_prefix: fn event -> "example_schema_prefix" end
end

The function will receive the event as the single argument allowing you to use the same or a different schema for each event.

Alternatively, you may now define a schema_prefix/1 callback function in the projector module:

defmodule MyApp.ExampleProjector do
  use Commanded.Projections.Ecto,
    application: MyApp.Application,
    name: "example_projection"

  @impl Commanded.Projections.Ecto
  def schema_prefix(event), do: "example_schema_prefix"
end

The function will receive the event as the single argument allowing you to use the same or a different schema for each event.

An example usage could be for tenant specific projections where each tenant's data is projected and stored in a separate database schema:

@impl Commanded.Projections.Ecto
def schema_prefix(%_{tenant: tenant}), do: tenant

Closes #27.