Add schema, validation and linting (for .ga and Format 2 in Python and Java)

[jmchilton]

• Implement manual custom linters for .ga (native) and Format 2 workflows in Python.
• Use schema-salad to write a schema for Format 2. Schema salad is a schema language built by Peter Amstutz et. al. for the common workflow language project. Using the same schema language - and indeed some of the same mixins/interfaces - continues Format 2 on a path toward convergence with the Common Workflow Language (where it makes sense).
• Schema-salad gives us both Python validation and nice HTML documentation for free. This work includes a script to generate Galaxy branded workflow documentation for Format 2 (leveraging More options to customize docs. common-workflow-language/schema_salad#298) and that validator.
• Use extensions to schema-salad I developed for this work (https://github.com/common-workflow-language/schema_salad/pull/299/files) to generate native Java packages for parsing/validation of Format 2 workflows as well.
• Use schema-salad generated Python parsing for Format 2 to enhance new linting of format 2 workflows.
• Extend new linting tests for .ga and Format 2 workflows to allow linting from native Java - re-implement identically tested linters in both languages. Should allow validation/linting from Dockstore using https://github.com/galaxyproject/dockstore-galaxy-interface.

TODO: This adds the framework but there is more linting and validation to be done - tracked in #14.