io.tnboot plugin should provide a vendor agnostic key vault integration

[testersen]

As a follow up to:

• feat: gradle plugin #10

The gradle plugin should provide an easy way to put secrets from a vendor agnostic key vault into environment variables before the app starts up.

There are multiple ways of doing this:

• In the gradle plugin itself.
• Injecting a Java Agent that adds it to properties.
• Or providing an SDK that can be used in the application.