Add middleware abstraction

[BjoernAtBosch]

Describe your changes

Add first version of middleware abstraction to get rid of C++ specific configuration (e.g. environment variables)

Issue ticket number and link

tbd

Checklist - Manual tasks

• Examples are executing successfully
• Created/updated unit tests. Code Coverage percentage on new code shall be >= 80%.
• Created/updated integration tests.
• Devcontainer can be opened successfully
• Devcontainer can be opened successfully behind a corporate proxy
• Devcontainer can be re-built successfully
• Extended the documentation (e.g. README.md, CONTRIBUTING.md, Velocitas)

[kse3hi]

General question: who is responsible for ENV_VARIABLE definition?
What about launch.json and tasks.json, is any modification is required?
Shall SDK get values not from ENV_VARIABLES directly, but by getting them out of the application context?

[BjoernAtBosch]

Answer to Serhii`s (kse3hi) general questions:

• Who is responsible for ENV_VARIABLE definition?
  -> The definer of the runtime.
• What about launch.json and tasks.json, is any modification is required?
  -> No modification required
• Shall SDK get values not from ENV_VARIABLES directly, but by getting them out of the application context?
  -> You mean the app context should parse the env vars? That would mean, that every app has to take care for it and need to handle middleware specifics.
  -> We do it "same" (from user perspective) way in Python SDK.

[doosuu]

LGTM