Improve HTTP requests and non-interactive authentication

[tadhg-moore]

Improved HTTP requests

Improved use of authorisation using keys, secrets and refresh tokens and updated packages used and

Details:

• Switched from httr to httr2 for all HTTP requests.
• A non-interactive authentication method that exchanges a long-lived refresh token for a short-lived Dropbox access token using the /oauth2/token endpoint enables use with Actions/Shiny apps etc.
• Accepts values directly or reads from DROPBOX_APP_KEY, DROPBOX_APP_SECRET, DROPBOX_REFRESH_TOKEN env vars\
• Converts expires_in → expires_at so the existing expiry-refresh logic in get_dropbox_token() works correctly on subsequent calls
• Preserves the original refresh token (Dropbox doesn't re-issue it)
• Stores the OAuth client so automatic token renewal continues to work
• Added cli package for improved user-facing messages, warnings, and errors.
• drop_search() upgraded to search_v2 with new filter params and pagination
  - drop_upload()now handles files >150 MB with the proper 3-step session upload API