Support age encryption during export and import

[ojarjur]

The export and import commands allow you to write/read a subset of the versioned files to/from a zip file.

That, in turn, is used by the push and pull helper for local files.

We should integrate with the age encryption tool to allow those zip files to be encrypted, and correspondingly update the rvcs-push-file and rvcs-pull-file extensions to support this new encryption feature.

[ojarjur]

This might not be feasible with zip file bundles, as the age decryption returns a streaming reader, but zip files need a random-access reader.

[ojarjur]

Alternatively, perhaps we should just support streaming import and export using compressed tar archives, and the user can just wrap that with age (or not) themselves.