Resolve "Option to force plaintext communication"

Description

Improve Server- and Client-side API to allow easy changing between secure and insecure communication

See #52 (closed)

What is the current behavior?

Users have no way to force insecure communication, it is just the last resort fallback option

What is the new/improved behavior?

• Users can explicitly force insecure communication.
• Secure communication is the default (on the server by either providing credentials or by letting the server create a self-signed certificate) and has no fallback to insecure communication as this is not useful (either you want to have encryption or you don't; there's no scenario where you would like to have encryption if possible, but you'd be fine if there's no encryption)

Does this MR introduce any breaking changes?

yes

• Server and Client c'tors have new signatures
• implicit fallback to insecure communication is removed

Other information

Checklist:

• The commit messages follow our guidelines
• If necessary, tests have been added
• Docs have been added / updated
• CHANGELOG.md has been updated in the Added/Changed section

Closes #52 (closed)