Make sandbox metadata editable

[ValentaTomas]

When you create a sandbox with metadata you cannot change this metadata later. Also, you cannot easily access this metadata on sandboxes without listing all sandboxes.

We should add endpoints for editing, deleting, and retrieving metadata (so it can be accessed on the sandbox and also on the reconnected sandbox) and SDK support for idiomatic manipulation with this metadata on the sandbox object.

The implementation might be supporting partial edits or only overwriting the whole metadata — this depends on the DX we decide to have.