feat: `dbListTables()` lists materialized views

[dpprdan]

This PR extends #413 and adds the queries of the system catalogs in order for dbList*() and dbExistsTable() to return information about materialized views in PostgreSQL. For background see #413 and #251.

In order to make the use of the system catalogs vis-a-vis information_schema.tables optional, I added a system_catalogs argument to dbConnect() (as suggested in #261). I set the default to TRUE (i.e. for regular PqConnections).

Redshift connections do not have the (explicit) argument, because there is (probably) no necessary extra information in it's system catalog views (i.e. above what's in information_schema). Since Redshift connections inherit from PqConnection, system_catalogs is set to FALSE for them.

I've added tests for materialized views, i.e. whether dbList*() and dbExistsTable() return information about matviews on the search path and on a custom schema.

closes #251

[krlmlr]

This now has conflicts.

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[dpprdan]

Thanks. I now disentangled and merged most preparatory PRs. Should we take another stab here?

🥳 I'll look into it, but again, it might take a couple of days until I'll get around to it.