Use AWS RDS IAM secrets from duckdb-aws

[staticlibs]

This PR builds on top of the following PR in duckdb-aws:

• Add RDS secret type for IAM auth duckdb-aws#144

With the RDS IAM token generation done in duckdb-aws, now two secrets
are required for IAM auth:

1. secret of type rds that contains all the details used to generate the IAM token
2. secret of type postgres that contains all other connection details except the IAM token; the rds secret name is specified in this secret

Example:

LOAD aws;
LOAD postgres;

CREATE SECRET aws_rds_secret1 (
  TYPE rds,
  PROVIDER credential_chain,
  PROFILE 'DatabaseAdministrator-<account_id>',
  CHAIN 'env;sso;',
  REGION 'eu-west-1',
  RDS_USER 'postgres',
  RDS_HOST 'database-1-instance-1.xxxxxxxxxxxx.eu-west-1.rds.amazonaws.com',
  RDS_PORT '5432'
);

CREATE SECRET pg_rds_secret1 (
  TYPE postgres,
  HOST 'database-1-instance-1.xxxxxxxxxxxx.eu-west-1.rds.amazonaws.com',
  PORT '5432',
  USER 'postgres',
  DATABASE 'postgres',
  SSLMODE require,
  AWS_RDS_SECRET aws_rds_secret1
);

Testing: tested locally with Aurora using both sso and env providers.

Ref: #464