Add postgres secret store

[aclowes]

The goal is to store the secrets in postgres so they can be retrieved remotely and automatically when attaching a lake. For example, the initial configuration is:

ATTACH 'postgres:postgresql://user:password@host/lake' AS metadata;
ATTACH 'ducklake:postgres:postgresql://user:password@host/lake' AS lake (DATA_PATH 's3://some-bucket');
CREATE SECRET IN postgres_metadata (
	TYPE S3,
	KEY_ID 'test_key_id',
	SECRET 'test_secret_value',
	REGION 'us-east-1'
);

After which, duckdb can be run in the JDBC driver for BI tools with a connection string like this, and it will be able to connect to postgres and S3 without further configuration:

jdbc:duckdb:ducklake:postgres:postgresql://user:password@host/lake

Feedback appreciated!

[aclowes]

@Mytherin thanks for triggering the build. I don't think the failures are related to my code; they are:

0.123 Initialized empty Git repository in /vcpkg/.git/
1.438 fatal: write error: No space left on device

and

OSError: [Errno 28] No space left on device: '/home/runner/work/duckdb-postgres/duckdb-postgres/build/reldebug/extension/json/json.duckdb_extension' -> '/home/runner/work/duckdb-postgres/duckdb-postgres/build/reldebug/repository/v1.4.0/linux_amd64/json.duckdb_extension'