support reading from multiple tables using logical replication slots

[maha-hajja]

Description

support reading from multiple tables using logical replication slots.

changes to params:

• "columns" param is removed.
• "key" param is updates now to accept multiple tables and keys, as a list of pairs, example: "table1:key1,table2:key2", if the key is not provided, we'll fetch the table's primary key.

Fixes #23

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[lovromazgon]

LGTM, there's a few followups we need to do, as you already pointed out in comments. I'll collect them here:

• update the snapshot iterator to fetch data from multiple tables
• remove key and column config fields
• think about actually having a key field that accepts comma separated table:column pairs
• later on introduce a Conduit-wide convention for specifying the source table/topic/collection that can be used for routing in destination connectors and add the metadata field instead of (or in addition to?) the postgres.table metadata field

I think at least the first three would be needed to make this feature complete so we can cut a release.

[lovromazgon]

One last thing.