#311 Add the ability to run multiple workflows in a single Spark Session

[yruslan]

Closes #311

The PR adds the following:

• Add the ability to run multiple workflows in a single Spark Sessions (--workflows command line argument)
  • Even if one of pipeline fails, Pramen will attempt to run the rest of the pipelines in the list.
  • The exit code will be non-zero if at least one of pipelines fail.
  • When running multiple pipelines from a library, you can access exit codes for each of executed pipelines in an list (see getExitCodes())
• Remove AppContext and AppContextFactory that become obsolete long ago and is not used.
• Make 'parallel.tasks' feature non-experimental.
• Fix email notification CSS style to remove IE gradient that is security risk.

[github-actions]

Unit Test Coverage

File	Coverage [90.55%]	🍏
TaskRunnerMultithreaded.scala	100%	🍏
PramenImpl.scala	100%	🍏
NotificationBuilderImpl.scala	100%	🍏
MetastorePersistenceTransient.scala	96.12%	🍏
CmdLineConfig.scala	95.1%	🍏
ConfigUtils.scala	92.09%	🍏
PipelineRunner.scala	91.3%	🍏
RunnerCommons.scala	83.26%	🍏
AppRunner.scala	80.95%	🍏
PipelineStateImpl.scala	79.95%	❌

Total Project Coverage	83.61%	🍏

[kevinwallimann]

lgtm