[MCP] Add custom key/value filtering support to Crashlytics MCP tools

## Summary

Feature Request: Add custom key/value filtering support to Crashlytics MCP tools (get_report and list_events).

The Crashlytics SDK allows developers to attach custom key/value pairs to crash sessions (e.g., user type, feature flags, A/B test variants, screen name). These custom keys are already returned as output on individual events (Event.customKeys), but there is currently no way to filter events or reports by custom key values through the MCP tools.

Proposed Change:
Add ability to filter using customFilters

Use Case
As a developer investigating crashes, I want to ask questions like:
- "Show me crashes where user_tier=premium"
- "How many crashes happened with feature_flag_x=enabled in the last 7 days?"
- "List crash events where screen_name=checkout"

Currently none of these queries are possible without fetching all events and filtering manually.