Add NewPostgresqlConnectorFromDSN() to pgutils

[andrew-corbalt]

PR Description

To simplify handling both RDS IAM auth and conventional password auth postgres connections, NewPostgresqlConnectorFromDSN() adds the concept of a custom DSN postgres+rds-iam://@[:]/, which will instruct the function to do down the IAM auth path in setting up a connector. Alternatively, if this function is passed a normal postgres DSN, then a conventional password auth is used.

Tests:
Updated the DSO Metrics Migration method to use NewPostgresqlConnectorFromDSN() and ran it against a test database using both conventional and custom DSN

PR Checklist

• New automated tests have been written to the extent possible.
• The code has been checked for structural/syntactic validity.
  • AMI/application: a build was performed
  • terraform changes: "terraform plan" checked on every affected environment
• (If applicable) the code has been manually tested on our infrastructure.
  • AMI/application: deployed an a test or dev environment
  • terraform changes: applied to test or dev environment
  • script: run against test or dev environment
• Likely failure points and new functionality have been identified and tested manually.
  Examples:
  • Application manually run in a way that triggers any new branches
  • AMI logged into and changes verified from login shell
• Pull request description includes a description of all the manual steps performed to accomplish the above.

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[jonahb]

This looks good. I like the new approach. I guess it'd also be worth thinking about whether we want to tweak things per Slack thread and how this change might fit into that.

[jonahb]

This function only accepts URLs (not any DSN / connection string), so I think we should make that clear, e.g.:

func NewPostgresqlConnectorFromURL(ctx context.Context, u *url.URL) (*PostgresqlConnector, error)

We could also provide a helper that parses the URL string (NewPostgresqlConnectorFromURLString).

[jonahb]

This seems good while clients are changing to connection strings (DSNs), but per engineering sync, the plan is eventually to remove the IAM and connection string-specific constructors.