Add new Connect bundle storage endpoints to SDK

[zackverham]

Three new admin-only API endpoints report bundle storage usage:

-   `GET /v1/system/storage`
-   `GET /v1/content/storage`
-   `GET /v1/content/{guid}/storage`

We should add support for these to the SDK.

[zackverham]

This maybe should wait to be implemented / released until after the new bundle storage apis are released though.