pgbk: specify the schema name in wal2json's add-tables

[espadolini]

We currently run queries on the kv table with no schema qualifier, which means that we will end up using the first existing schema in the database's search_path, both when creating or updating the tables, and when using them. This can be used by database administrators to have Teleport use a schema that's not the default public schema, but the wal2json change feed was working under the assumption that the schema was always public, leading to a confusing scenario where no error is signaled, but the change feed pulls no events because it's looking for changes in public.kv when the actual table is some_other_schema.kv.

With this PR we read the schema name for the (unqualified) kv table when we open the change feed connection, and pass it to add-tables later. The string quoting strategy for the wal2json option (prepending a backslash to every character) was inspired by https://github.com/supabase/walrus/blob/aa36cf9ac8667573bc109fd62d4a57f267131b42/sql/walrus--0.1.sql#L125 .

changelog: fix: the PostgreSQL backend now handles default database schemas other than public