fix return type of `client.CycloneDxCmd.execute` and use of `exit`

[jkowalleck]

the return type of cyclonedx_py.client.CycloneDxCmd.execute is currently None.
unfortunately this is not true. There are cases in which the method will never return, but exit python. there are other cases when the method returns. Therefore the return is sometimes typing.NoReturn

motivation:

• be able to write unit tests for cyclonedx_py.client.CycloneDxCmd.execute
• be able to catch raise conditions/exit codes in unit tests.

Proposal:

• change the method to a bare state where it will always exit, but never return.
  therefore use sys.exit() and return type of typing.NoReturn
  calling methods - like unit tests - could catch the SystemExit exception and work with it if needed/
• cyclonedx_py.client.main() should have a return type of NoReturn as well.
• do not use exit but sys.exit - with proper ext message or raise condition.

acc crit:

• all methods & functions of cyclonedx_py.client either return or exit. they may throw exceptions in addition.

alternative proposal or an addition:

• implementation detail: use click for arg parsing and CLI foo

[jkowalleck]

v4 will remove this API.
fixed via #605

[jkowalleck]

This feature will be part of the next/upcoming major release.
Changelog: see #605
Install via: pip install cyclonedx-bom==4.0.0rc1