Add task tags to app metadata and populate in view signing

[keighrim]

Because

With appTags defined in the MMIF spec (clamsproject/mmif#247) and implemented in mmif-python (clamsproject/mmif-python#375), clams-python needs to:

1. Let app developers declare task tags in their app metadata
2. Propagate those tags into view metadata when signing views

Done when

• appTags (or tags) field on AppMetadata
• sign_view() writes appTags from app metadata into view.metadata.appTags
• Tests

Additional context

this is to implement the decision made in #262