The method for `RoleDescriptor` parsing is becoming more complex due to
its usage being shared between API keys, file-based and native roles.
In some cases we allow 2.X format, in others we disallow
restrictions. Having a single method with multiple boolean flags that
control inclusion/exclusion of fields is becoming hard to extend.
This refactoring aims to allow easier introduction of new fields that
should be conditionally supported in some cases but not others.
One of such cases is introduction of `description` field that should only
be supported for file-based and native roles but not for roles embedded
in API keys.

…tor-role-parsing

# Conflicts:
#	x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authz/store/FileRolesStore.java

…tor-role-parsing

…very time