Collectors: Mark optional form fields in source forms

## Context

Split from Graylog2/graylog-plugin-enterprise#13650 (sub-issue 4).

## Problem

In the Collectors source form (`SourceFormModal.tsx`), optional fields are not explicitly labeled. The `Name` field is marked `required`, but fields like `Description` and type-specific optional fields (e.g., Journald `Match Pattern`) have no "(Optional)" indicator. Users cannot tell at a glance which fields are mandatory.

## Suggested Fix

Mark optional fields with an "(Optional)" label, or consistently mark required fields with an asterisk — whichever pattern the rest of the Graylog UI follows.

## Relevant Files

- `graylog2-web-interface/src/components/collectors/sources/SourceFormModal.tsx`