Add the `x` flag to properties

[andamian]

We started with the vault implementation where it didn't make sense to have it but in arc does. I should add it to arc but implement it as a property in vault as well so the flag is always preserved on the round trips (local->hospice->local). (Reference: opencadc/vostools#212)

[pdowler]

I assume this is for files as the x flag for directories is part of "read permission": ~permission to list. Full read permission is thus r-x for dirs.

The current code should preserve x flags set by users in the platform (eg with chmod on the filesystem).

[pdowler]

To round trip and retain a x flags, we would have to define property URIs(). There are technically 3 x flags (owner, group, world) so execute, group-execute, world-execute would be the complete solution.

[andamian]

This also needs to be propagated to the client when available.