feat: add support for Postgres IAM authentication in JDBC and R2DBC connectors

[shubha-rajan]

Change Description

Adds IAM Auth support. Some notes to keep in mind:

• Users need to include a nonempty string for the password when connecting. This will be ignored, but is required to bypass driver errors for null and empty passwords.
• sslmode needs to be disabled or the driver will hang indefinitely.

Checklist

• Make sure to open an issue as a
  bug/issue
  before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea.
• Ensure the tests and linter pass
• Appropriate documentation is updated (if necessary)

Relevant issues:

• Fixes Add support for IAM database authentication #350

[kurtisvg]

We already have a credential socket factory right? Can we get a token source from the credentials?

[kurtisvg]

I guess what I'm saying is we already have a property for setting the user credentials, can we reuse the user credentials instead of trying to ask for the token source from somewhere else?

[shubha-rajan]

So the Credential Factory returns credentials that are wrapped in a HttpCredentialsAdapter and there's no way to get credentials back from that, because they're private https://github.com/googleapis/google-auth-library-java/blob/master/oauth2_http/java/com/google/auth/http/HttpCredentialsAdapter.java#L64

We could change the Credential Factory to return a GoogleCredentials object and then convert that to the HttpCredentialsAdapter later, but I'm hesitant to change the existing interface

[kurtisvg]

That's a DPE owned library - can we PR a getter so we can have access to it?

[shubha-rajan]

googleapis/google-auth-library-java#658

[meltsufin]

@shubha-rajan @kurtisvg

sslmode needs to be disabled or the driver will hang indefinitely.

Are there any security implications to disabling sslmode?

[kurtisvg]

@meltsufin No - there are two potential layers of encryption - at the connection level, and at the database protocol level. Disabling at the database level is fine because the connection level is always persistent.