sql_database: integrates sql database source (Feat/1671)

[willi-mueller]

Description

• adds the implementation
• adds tests run locally on PostgreSQL & duckdb into tests/sources
• extracts tests run on all destinations into tests/load/sources
• formats code and imports
• corrects example PostgreSQL connection string for the test suite (postgres:// -> postgresql://)

Related Issues

• Resolves https://github.com/dlt-hub/dlt/issues/1671

TODO before merge

• [x] Decide how to proceed with sqlalchemy incompatibility. Our test suite requires 2.0 but airflow requires <2.0
• [ ] check if postgresql connection secret needs to be adapted on CI
• [x] rebase on latest changes from devel branch

[netlify[bot]]

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