Add und use schema for YAML task-definition files

[PhilippWendler]

A formal definition of what is allowed in task-definition files would be helpful for humans to understand the format, and could allow automatic validation of such files.

Beause YAML is a super set of JSON, it is probably best to use JSON Schema as schema language. An example how this schema would look like can be seen here for an older version of task-definition files or in svcomp-build-mockup and symex-fp-bench.

[PhilippWendler]

The issue for defining a formal schema is now at https://gitlab.com/sosy-lab/software/task-definition-format/-/issues/1.

After this is available, we should consider using it in BenchExec to validate task-definition files when opening them.