fix: cleaned up and rewritten execution backend structure

[johanneskoester]

Description

• fixes Environment variables are not correctly quoted when executing job on cluster #860
• fixes Environment variables with spaces break execution on clusters when using jobscript #1007
• replaces Protects against spaces in env vars and avoid quoting of all env vars as single entity #1008 with a global solution
• fixes Incompatibility between Snakemake (v7.0.0 or later) and Tibanna?  #1475

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
• The documentation (docs/) is updated to reflect the changes or this is not necessary (e.g. if the change does neither modify the language nor the behavior or functionalities of Snakemake).

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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