feat: add support for service account impersonation

[enocom]

This is a port of GoogleCloudPlatform/cloud-sql-proxy#1460.

[hessjcg]

See if you can get rid of the redundant switch statement.

[hessjcg]

Seems like this switch statement from line 130-150 are much identical to lines 166-189. Could you somehow
combine those into a separate function?

iopts := c.withCredentialSource(l) // switch statement in here
if c.ImpersonateTarget != "" {
   ts, err := impersonate.CredentialsTokenSource( ... iopts)
   // ... error handling, etc
   return alloydbconn.WithTokenSource(ts)
}
return iopts

[kurtisvg]

If we are making significant changes to this PR we should make sure to update the Cloud SQL go connector as well

[enocom]

Let's fix this in the v2 proxy and port the change here.

We'll land this change in the Cloud SQL proxy and port it back over here.