Cache task properties

[budius]

What does this PR introduce?

• Reworked the Gradle plugin and Task to proper use the input/output and operate seamless with the gradle caching.
• Also properly connect the generated code task with the compilation process so that it is automatically executed.

Resolved or linked issues?

Fix #15

Notable Changes

• Reworked the gradle extension to be interface and add the appropriate annotations
• extracted the gradle task to its own GenerateKtorServer class
• structured the applyGenerateKtorServer following best practices.

[rvp-diconium]

LGTM