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.

Start with the plugin settings tab and DQLSyntaxSettingTab in src/main.ts to understand the existing Obsidian UI and settings flow. Then determine how schema data, local caching, and optional Dynatrace API access fit the proposed design. Done means users can browse data sources and fields, filter them, and insert a field into the current DQL block.

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