Prefix support

[sveredyuk]

Hi, I am actively using PG schemas and prefix options for Repo.insert(%{}, prefix: "my_shema") in my current multi-tenant application. Any plans on support the prefix option for schema? I can contribute if you are open.

Currently, I can add versions inside every schema, and it works fine on insert and update, but for delete, I got the trouble:

** (Postgres.Error) ERROR 42P01 (undefined_table) relation "versions" does not exist

    query: INSERT INTO "versions" ("account_id","action","entity_id","entity_schema","id","patch","recorded_at") VALUES ($1,$2,$3,$4,$5,$6,$7)
    (ecto_sql 3.6.2) lib/ecto/adapters/sql.ex:760: Ecto.Adapters.SQL.raise_sql_call_error/1
    (ecto_sql 3.6.2) lib/ecto/adapters/sql.ex:667: Ecto.Adapters.SQL.insert_all/9
    (ecto 3.6.2) lib/ecto/repo/schema.ex:58: Ecto.Repo.Schema.do_insert_all/7
    (ex_audit 0.9.0) lib/repo/schema.ex:72: anonymous fn/4 in ExAudit.Schema.delete/4
    (ex_audit 0.9.0) lib/repo/schema.ex:155: ExAudit.Schema.run_in_multi/4
    (ecto 3.6.2) lib/ecto/multi.ex:716: Ecto.Multi.apply_operation/5
    (elixir 1.12.1) lib/enum.ex:2356: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto 3.6.2) lib/ecto/multi.ex:690: anonymous fn/5 in Ecto.Multi.apply_operations/5
    (ecto_sql 3.6.2) lib/ecto/adapters/sql.ex:1017: anonymous fn/3 in Ecto.Adapters.SQL.checkout_or_transaction/4

Maybe somehow it's possible to disable delete tracking for now? I can also contribute these options as well.

p.s. Thanks for doing this amazing open-source solution that should simplify my work a lot