nodemailer caches smtpTransport singleton

[mschipperheyn]

Describe the bug
the nodemailer implementation stores smtpTransport on the instance. This invalidates the getTransport call if you are in a multivendor scenario and makes it impossible to have vendor specific smtp configurations.

My colleage Ankish identified this issue and is adding a PR for it

To Reproduce
Steps to reproduce the behavior:

1. Create mutliple channels with their own smtp configs
2. Select the config on the fly
3. Only the first used transport gets used

Expected behavior
Transport should be selected on the fly based on configuration settings.

Environment (please complete the following information):

• @vendure/core version: 3.0.8
• Nodejs version
• Database (mysql/postgres etc):