Add support for IAM database authentication to Amazon RDS

[sjauld]

Is your feature request related to a problem? Please describe.
Passwords are bad. Identity-based authentication is good. If the cube postgres/maria/mysql drivers supported IAM authentication that would be nice.

Describe the solution you'd like
Just add some env variable like CUBEJS_DS_POSTGRES_DB_USE_AWS_IAM=true and it magically works.

Describe alternatives you've considered
Stick with using passwords and have to stuff around with rotation and distribution of credentials etc.

Additional context
RDS IAM auth docs

[igorlukanin]

@sjauld Great idea! Would you be open to contribute a PR?

[github-actions]

If you are interested in working on this issue, please go ahead and provide PR for that.
We'd be happy to review it and merge it.
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