Add public getDsn method to adapters for unit testing constructed DSN strings

[MasterOdin]

For each adapter, there's a handful of ways to customize the DSN string we generate that is passed to PDO. However, there's not currently a good way we have to test that said behavior is correct / good and so we have some nonsensical tests like:

https://github.com/cakephp/phinx/blob/48d8e1f686852882a319e6deff1ae6748d04ab5c/tests/Phinx/Db/Adapter/SqlServerAdapterTest.php#L62-L68

Was unsetting the port meaningful? Who knows!

While we could maybe get what we want out of some level of reflection / mocking around the protected createPdoConnection method, that should be avoided if possible.

Instead, having a getDsn method that returns a string instead would more easily solve the problem, and allow for best practices around unit testing these things, and ensure behavior is good and correct. It's possible that said method could also be useful to someone downstream, but I'm not overly concerned at the moment with that.