Accept :memory: mode for credentials parameter in duckdb factory

[sultaniman]

This PR addresses #1216 and allows duckdb(:memory:) request

TODO

• Tests

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[rudolfix]

OK it looks like we have problem with documentation, not the code. We clearly say how to pass in memory duckdb to be used:

The destination accepts a duckdb connection instance via credentials, so you can also open a database connection yourself and pass it to dlt to use. :memory: databases are supported.

import duckdb
db = duckdb.connect()
p = dlt.pipeline(
  pipeline_name='chess',
  destination=dlt.destinations.duckdb(db),
  dataset_name='chess_data',
  full_refresh=False,
)

but then say the following:
A few special connection strings are supported:

• :pipeline: creates the database in the working directory of the pipeline with the name quack.duckdb.
• :memory: creates an in-memory database. This may be useful for testing.

what we should IMO do

1. drop :memory: form docs
2. instead of accepting :memory: let's raise an exception that says what to do instead

if we create the duckdb instance in memory, it will be lost after the data loading, right? even if we somehow avoid closing it, passing this as external db is IMO way more intuitive

[sh-rp]

one small suggested change, but it's good!
and one request: please don't mark comments as resolved, the commenter should do this once they are satisfied that you have made the necesary change

[rudolfix]

code looks good but documentation was only partially updated and IMO not reviewed before push.

thing that helps me is to read my diff in the PR before asking for a review. because now we need another round and the ticket is delayed again...

[rudolfix]

LGTM!