feat: Add Scaffolding to CLI

[amritanshusikdar]

Description
This PR aims to have a more reliable project structure for the CLI and move the scaffold feature from the regular Kyma CLI.

Related issue(s)
Resolves #9

Aspects to consider for knowledge sharing

• business logic (internal/scaffold) is decoupled from command logic (cmd/modulectl/create/scaffold)
• business logic invocation has a clear contract (internal/scaffold/options.go)
  • validation has been shifted left (previously flags were validated after creating the object, now at the beginning of the business logic invocation)
• business logic is unit tested, overall command logic is covered by e2e tests
  • we decided to not introduce integration tests in addition as the unit tests and e2e tests should already be sufficient
• comments on SOLID:
  • S: e.g., dedicated services (file generator, content provider, ...) each having a single responsibility
  • O: e.g., FileGenerator and ReuseFileGenerator (decorator pattern), where the later adds functionality on top of the former without modifying the former's code
  • L: n/a
  • I: e.g., ScaffoldService and (Reuse)FileGenerators, each defining the exact dependencies as interfaces they rely on
    • e.g., FileGenerator requiring a FileWriter, while ReuseFileGenerator requires a FileReader (+ the generator)
  • D: e.g., all services, dependencies are injected into constructor method allowing easy and extensive unit testing (see also the test stubs)
• guard clauses used to prevent developer errors when composing the functionality

[CLAassistant]

All committers have signed the CLA.

[mmitoraj]

The docs look great. I've added 3 suggestions; however, I wasn't sure where to add them. Are those autogenerated .md files? Where is the source? As far as I know, the autogenerated docs cannot be edited, I usually added comments for those to the respective .go or .yaml files - not sure which ones. Does it work the same here?