[#190]: Add upsert support via onConflict argument

## What Implements upsert support via `onConflict` argument on insert mutations. Closes #190 ## Why This enables upsert operations (INSERT ... ON CONFLICT DO UPDATE) through the GraphQL API, which is a common requirement for applications that need idempotent writes. ## Changes - **SQL Context**: Load index names from `pg_class` to identify constraints - **SQL Types**: Add `on_conflict_indexes()` and `has_upsert_support()` methods to Table - **GraphQL Types**: - New `{Table}OnConflict` input type with `constraint`, `updateColumns`, and `filter` fields - New `{Table}Constraint` enum (unique index names) - New `{Table}UpdateColumn` enum (updatable column names) - Conditionally expose `onConflict` arg only on tables with valid constraints - **Builder**: Parse `onConflict` argument and map GraphQL column names back to Column objects - **Transpile**: Generate `ON CONFLICT ON CONSTRAINT ... DO UPDATE SET ... WHERE ...` ## Example ```graphql mutation { insertIntoAccountCollection( objects: [{ id: 1, email: "new@example.com", name: "Updated" }] onConflict: { constraint: account_pkey updateColumns: [email, name] filter: { status: { eq: "active" } } } ) { affectedCount records { id email name } } } ``` ## Notes - Tables with serial/generated primary keys will not expose `onConflict` (constraint columns must be insertable) - Empty `updateColumns` results in `DO NOTHING` - Based on #503 but reimplemented on current master without unrelated changes ## Tests Added tests in `test/sql/mutation_insert_on_conflict.sql`