add more helpers for counts

Author: hayes

.changeset/funny-hoops-shine.md

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+---
+"@pothos/plugin-drizzle": minor
+---
+
+Add new field builder methods and connection enhancements:
+
+- `t.relatedField`: New method for creating custom fields based on relations with custom selections using the `buildFilter` callback
+- `t.relatedCount`: Simplified method for counting related records, with optional `where` filter support
+- `totalCount: true` option for `relatedConnection`: Adds a `totalCount` field to related connections
+- `totalCount` callback for `drizzleConnection`: Allows adding a `totalCount` field to drizzle connections
+- `totalCountOnly` optimization: Skips the main query when only `totalCount` is requested (for both connection types)