model executor for postgres and mysql to duckdb

[k-anshul]

subtask for https://github.com/rilldata/rill-private-issues/issues/854

• Adds a model executor that ingests data from postgres/mysql using duckdb extension.
• Also supports incremental ingestion.

Sample model yaml that ingests from postgres.

connector: postgres
sql: SELECT * FROM students {{ if incremental }}
database_url: "postgresql://postgres:postgres@localhost:5432/postgres"

output:
  connector: duckdb

Backward incompatible change

1. The MySQL duckdb extension does not support URI type dsn so users will need to submit dsn in key=value form. Have raised this issue with duckDB.
2. Will update docs once sources are fully removed.