Protocol support for packfiles

## Is this a feature request or a bug?
Specification feature.

## Expected behavior:
The specification supports a structure analogous to Git's packfiles - a collection of content-addressable files frequently downloaded together. This delivers cross-file compression, saving precious network bandwidth.

If a pack file has not yet been generated for a download, the client can fall back to individual file downloads or the previous pack file with additional individual downloads to fill the gaps.

Pack file locations are delivered on a "best-effort" basis with no cryptographic integrity. The actual pack files are content-addressed. Clients MUST verify that the pack contents are the files they wanted.

## Actual behavior:
The diagram indicates that all files must be downloaded individually.

## Environment info (where relevant)
Specification version https://github.com/entropic-dev/entropic/blob/0f864e1df8a2bbe323910af9e79b249be29192a1/docs/README.md