feat: Add roles-separator to split roles from attributes

[frehov]

closes #787

[sdelamo]

what do you think if we rewrite this method as:

@NonNull
    private List<String> rolesAtObject(@Nullable Object rolesObject) {
        if (rolesObject == null) {
            return Collections.emptyList();
        }
        if (rolesObject instanceof CharSequence && tokenConfiguration.getRolesSeparator() != null) {
            return Arrays.asList(rolesObject.toString().split(tokenConfiguration.getRolesSeparator()));
        }
        if (rolesObject instanceof Iterable) {
            List<String> roles = new ArrayList<>();
            for (Object o : ((Iterable<?>) rolesObject)) {
                roles.add(o.toString());
            }
            return roles;
        }
        return Collections.singletonList(rolesObject.toString());
    }

Moreover we have to update the Javadoc to explain the change

[frehov]

This is what I should've done.
I do prefer static imports from Colleactions, as it mostly helps with readability, but I'm not sure if that's allowed by the checkstyle

[sdelamo]

I am not sure about the copy. This shows in our docs. I would like to convey it is used if the roles is a string.

What do you think if we change it to something like the following?

If the entry used for the roles in the {@link io.micronaut.security.authentication.Authentication} attributes map is a String, you can use the separator to split its value into multiple roles. Default value {@value io.micronaut.security.token.config.TokenConfiguration#DEFAULT_ROLES_SEPARATOR}.

[frehov]

I'm with you on that, it conveys the intention better