Fix/set tokens

[HJordan35]

Please explain how to summarize this PR for the Changelog:

The FlatfileProvider supports 2 props:

• publishableKey
• accessToken
  When attempting to create NEW spaces, the publishableKey must be provided (accessKey must be null) - however, when re-using a space, and accessToken must be provided, while leaving publishableKey blank. However, if there is ever a need to support BOTH situations simultaneously (i.e. sometimes create a new space, sometimes re-use) there is not efficient way to update the FlatfileProvider

This update exposes the setAccessToken and setPublishableKey props - exposed via the useFlatfile hook so customers with this unique requirement can patch their provider depending on the use-case, without needing to perform a complete unmount/remount of their provider

Tell code reviewer how and what to test:

[promptless]

✅ No documentation updates required.