Add integrity support to FileSystemSecurityRealm

This follows up on ELY-2078, which added support for encryption to the filesystem realm.

This RFE is to track adding support for integrity to the filesystem realm.

If a realm gets large it would be expensive to recursively check the integrity of every identity in the realm so instead maybe an individual entry should have it's own signature when written and a master index then signed to represent the presence of each identity.  The entries should likely be versioned with the version a part of the master index to prevent an older entry being swapped back in.

Signing of sections of an identity or the complete identity could be considered.