Add roles to common metadata #1267

[m-mohr]

Related Issue(s): #1267

Proposed Changes:

1. Add roles to common metadata, currently used in Assets, Providers, Contacts extension, and planned to be used in the CEOS ARD and Web Map Links extension for links.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG
  or a CHANGELOG entry is not required.
• This PR affects the STAC API spec,
  and I have opened issue/PR #XXX to track the change.