Provide an API to show "all tahoe object-sizes" in magic-folder

[meejah]

In order to get good pricing estimates from ZKAPauthorizer it needs to know how many and of what size every Tahoe capability magic-folders currently cares about.

This means returning, per magic-folder, a list of integers. For each Snapshot, that means: the size of the "content" capability, the size of the "metadata" capability and the size of the Snapshot capability itself.

[meejah]

I know we talked about estimating these, but with a couple updates to the database we can be exact since for all remote-snapshots we have the 3 capabilities (the Snapshot cap, metdata cap and content cap). We currently don't cache the latter two.

(Capabilities contain their exact size)

[tomprince]

@crwood Would it be more useful to have the list of sizes of things we have already uploaded to the grid, Or the sizes we will have uploaded[1], once everything is done syncing (assuming no changes happen in the interim?

[1] Ignoring, I think intermediate snapshots; just the size of the reachable objects at that time.

(There is some more discussion in #524 (comment))

[crwood]

@tomprince I'm thinking it would be more useful to have the list of filesize for objects that have already uploaded to the grid (since this API will be used to determine the user's projected "monthly ZKAP cost" for all files currently stored in the grid).

[tomprince]

@crwood If we are looking to get the projected monthly cost, it seems like including stuff that we are about to upload will give a better projection of that, than the current file sizes; at least on an ongoing basis. Unless the existing snapshots are renewed before we upload new ones (for which there is definitely a window), the user won't be charged for file already on-grid that we have newer snapshots for.