feat(dropbox): add root_namespace_id to access teams folder

[shouko]

Just started using this tool recently, it's a great project!
Still not very familiar with the codebase or golang, feel free to provide feedback and suggestions.

It's been quickly tested with my Dropbox Business account manually, now it's able to see teams folders.

Background

Related to #5126 and #5760

Sending Dropbox API request without a Dropbox-API-Path-Root header defaults to user home namespace.
To access Dropbox Team Space, proper namespace needs to be provided.

Namespace IDs can be obtained from /2/users/get_current_account.
For non-business users, root_info.root_namespace_id and root_info.home_namespace_id seem to be identical values.

For Business users, setting root_namespace_id in Dropbox-API-Path-Root header allows API client to start from a higher level root where the requesting user's personal home as well as other shared folders can be seen at the same time.

Changes

• Makes a request to get namespace IDs in Init
• Set Dropbox-API-Path-Root header in request method when d.RootNamespaceId is available

References

• https://www.dropbox.com/developers/documentation/http/documentation#users-get_current_account
• https://developers.dropbox.com/en-us/dbx-team-files-guide#interacting-with-team-content
• https://qiita.com/nkmk1215/items/95e2d97e5bb89a8b904a
• https://github.com/rclone/rclone/blob/810644e8735330db1cbd70745802284e8624b423/backend/dropbox/dropbox.go#L184

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