feat: Add support for Azure secrets

[Y--]

Add support for Azure secrets:

1. Install Azure extension

SELECT duckdb.install_extension('azure');

2. Add a secret:

INSERT INTO duckdb.secrets
(type, connection_string)
VALUES ('Azure', '<your connection string>');

3. Run a query:

SELECT count(*) 
FROM read_parquet('azure://my_container/orders.parquet') AS (order_id int);

[JelteF]

Looks good overall. Some small nitpicks.

[JelteF]

I think we should call out somewhere that writes to Azure are not yet supported, linking to this issue: duckdb/duckdb-azure#44

[Y--]

Added a note below in the readme, let me know what you think.