Migrate to new Go client library credentials

[enocom]

The oauth2 package is no longer the preferred choice for token sources. Instead, we should use:

https://pkg.go.dev/cloud.google.com/go/auth#Credentials

This means adding a new API option, replacing the internals with auth.Credentials, and directing users away from our current WithTokenSource and related options that use oauth2.

Also we should be using https://pkg.go.dev/cloud.google.com/go/auth@v0.13.0/credentials#DetectDefault.

This is a companion to GoogleCloudPlatform/cloud-sql-go-connector#904.

[enocom]

cc @codyoss @quartzmo Here's the AlloyDB issue tracking migration to the new auth package.

[jackwotherspoon]

Just an FYI: Don't be like me and forget to set headers for quota project and user agents:

GoogleCloudPlatform/cloud-sql-go-connector#920

[enocom]

We might want to do this as well: GoogleCloudPlatform/cloud-sql-go-connector#997.