Add example with external tasks

[ingorichtsmeier]

Description

Add a Camunda 7 Example application with external service task. And migrate it using the camunda-7-adapter.

Additional context

Some people are struggling with the correct configuration and dependencies. And we have no running example right now.

Testing your changes

Both applications, Camunda 7 and migrated, contain unit tests to run and assert process instances.

Types of changes

• Bug fix (non-breaking change which fixes an existing open issue)
• New feature (non-breaking change which adds functionality to an extension)
• Breaking change (fix or feature that would cause existing functionality of an extension to change)
• Documentation update (changes made to an existing piece of documentation)

Checklist:

• My code adheres to the syntax used by this extension.
• My pull request requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the Camunda Community Hub documentation.
• I have read the Pull Request Process documentation.
• I have added or suggested tests to cover my changes suggested in this pull request.
• All new and existing CI/CD tests passed.
• I will /assign myself this issue, and add the relevant [issue labels] to it if they are not automatically generated by Probot.
• I will tag @camunda-community-hub/devrel in a new comment on this issue if 30 days have passed since my pull request was opened and I have not received a response from the extension's maintainer.

[github-actions]

Test Results

  25 files  +  25    25 suites  +25   1m 42s ⏱️ + 1m 42s
221 tests +221  221 ✔️ +221  0 💤 ±0  0 ❌ ±0 
337 runs  +337  337 ✔️ +337  0 💤 ±0  0 ❌ ±0 

Results for commit 14a3472. ± Comparison against base commit 6137a4f.

[jonathanlukas]

Looks good to me!