Package reboot

[kevinykuo]

This PR is a reboot of the package with major API changes. Usage examples can be found in the README. The main design decisions are as follows:

• In naming functions, we use noun_verb() throughout, where noun (always singular) tells the type of the first argument and not necessarily the REST endpoint. The space listing function has been renamed to rscloud_space_list().
• Since the space concept is the entry point into most of the functionality, it has been implemented as an R6 class (primarily to maintain a status attribute).
• Authentication now uses environment variables (RSCLOUD_CLIENT_ID and RSCLOUD_CLIENT_SECRET) instead of a config.yml in the project directory. Secrets are no longer cached in package globals, and spends a minimal amount of time in process memory. (Closes #11)
• Authentication is now implicit, i.e. users don't need to invoke a specific function to get authenticated. (Closes #23)
• For testing, the API URL and host can be set via the rscloud_{api_url/host}_set() functions or by setting the environment variables RSCLOUD_API_URL and RSCLOUD_HOST. We can re-introduce a config file in the future for specifying e.g. instance sizes.
• The features suppported in this change include
  • Spaces
    • List
  • Space members
    • List
    • Add
    • Remove
  • Space roles
    • List
  • Invitations
    • List
    • Resend
    • Rescind
  • Projects
    • List
  • User
    • whoami (Closes #12)
• Tests have been added (Closes #22)

[kevinykuo]

@coatless @mine-cetinkaya-rundel would be great if y'all can give this a whirl sometime!

[coatless]

Quick glance over the restructured code, I think the invitation_send() functionality will be confused w.r.t. space_member_add(). Perhaps the earlier function could be renamed?

I'll dig a bit deeper tomorrow.

[kevinykuo]

I think this is pretty well marinated, merging!