Add Dynatrace schema explorer for data source discovery

Writing effective DQL queries requires knowledge of available data sources and fields. Adding a schema explorer panel would help users discover what data they can query.

**Proposed Changes:**
- Create a sidebar panel showing available Dynatrace data sources (logs, metrics, events, etc.)
- Display field names and types available in each data source
- Allow filtering/searching for specific fields
- One-click insertion of field names into the current DQL block
- Cache schema data locally to minimize API calls
- Optional Dynatrace API integration to fetch live schema from the user's environment

**Benefits:**
- Users can explore data without leaving Obsidian
- Reduces need to switch between Obsidian and Dynatrace platform
- Helps newer users learn about available data sources
- Improves query accuracy by showing valid field names

**Related Code:**
Would require extending the plugin settings tab (`DQLSyntaxSettingTab` in `src/main.ts`) and potentially adding API integration code.