Block users

[gmgigi96]

This PR allows an operator to set a list of users (username) that are banned for every operation in reva.
This requires that every service is restarted to get the new list if a user is added!

Example:
Given the storage-references example.
Add in gateway.toml:

[shared]
blocked_users = ["einstein"]

$ cmd/reva/reva -host localhost:19000 -insecure
reva-cli v1.19.0-124-ga4ffa8aa (rev-a4ffa8aa)
Please use `exit` or `Ctrl-D` to exit this program.
>> login basic
username: einstein
password: error: code=CODE_PERMISSION_DENIED msg="user is blocked" support_trace="ac860997b02743ef52b9d5bdab4311d0"

Even with a valid token:

>> whoami
rpc error: code = PermissionDenied desc = user einstein blocked

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[labkode]

@gmgigi96 looks good, can you just add an example on the config change?