Extra CRAN metadata for pak and pkgcache

Currently contains:

• file sizes,
• SHA 256 hashes,
• system requirements, i.e. the SystemRequirements field from DESCRIPTION,
• for binaries, their build timestamp, i.e. the Built field from DESCRIPTION,
• the timestamp when the package was published on CRAN, i.e. the Date/Publication field from DESCRIPTION.

License

MIT (c) Posit, PBC