Add support for lightweight user types

[ishank011]

This PR adds support for assigning and consuming user type when setting/reading users. On top of that, support for lightweight users is added. These users have to be restricted to accessing only shares received by them, which is accomplished by expanding the existing RBAC scope.

In EOS, ACLs for such users cannot be persisted, so for keeping track of the allowed permissions, we maintain shares with lightweight accounts in an extended system xattr sys.reva.lwshare and these are enforced by minting tokens for such accounts allowing access to the shared resource on behalf of the resource owner

Test requests:

$ curl --location --request POST 'http://localhost:19001/ocs/v1.php/apps/files_sharing/api/v1/shares?shareType=0&path=/abc&shareWith=lwaccount' -u einstein:relativity # returns 200
$ curl -k GET 'http://localhost:19001/ocs/v1.php/apps/files_sharing/api/v1/shares?shared_with_me=true&state=all' -u lwaccount:lightweight # returns 200
$ curl -k -X PROPFIND http://localhost:19001/remote.php/webdav/reva/einstein/abc -u lwaccount:lightweight # returns 207
$ curl -k -X PROPFIND http://localhost:19001/remote.php/webdav/reva/einstein/pqr -u lwaccount:lightweight # returns 500 TODO: should return 403 (need to fix ocdav response codes)
$ curl -k -X GET http://localhost:19001/remote.php/webdav/reva/einstein/abc/a.txt -u lwaccount:lightweight # returns 200