Add a possibility to specify default values for fields in schema salad

[hmenager]

It would be interesting to be able to specify a default value for fields in schema_salad based specs. Currently they are mentioned in the documentation field, but this is not really machine readable (e.g. https://github.com/common-workflow-language/common-workflow-language/blob/master/v1.0/CommandLineTool.yml#L172). Having these would help a lot to generate SDKs ;)
cc @anton-khodak @khillion

[tetron]

1. Add "default" field to Salad metaschema
2. Add support to Python codegen to set value of "default" on fields which are not loaded from document
3. Update CWL spec with machine-readable defaults

[mr-c]

2020-12-02 update:

1. default was added to the schema-salad v1.1 spec: https://www.commonwl.org/v1.1/SchemaSalad.html#Introduction_to_v1.1
2. No updates have been made to the Python codegen yet about defaults (nor the Java codegen)
3. There are many defaults set in CWL v1.2, here's PR to add 4 more: More defaults cwl-v1.2#68