Document optional and mandatory fields in config.json file and validate it

[manojrupireddy]

For better developer experience, it will be good add on to document what are the optional fields and what are mandatory fields in config.json file here.https://github.com/deislabs/mystikos/blob/main/doc/sign-package.md . If it is not already part of the code, it would be good to add validation checks around this during packaging.

[paulcallen]

at the same time we may want our myst sign-sgx and myst package-sgx option to generate a clean sample config.json which also has comments to say what defaults are

[paulcallen]

another tool that may be useful is for appbuilder scenarios where we may be able to generate a config.json based on the options inside the docker file