Add postgres configuration store for on-demand schema loading

[manasag]

Summary

• Adds alternate configuration mode where the connector reads collection schemas from a PostgreSQL config store (config_tables.raw_schema) instead of filesystem JSON files
• Configuration is fetched per-request rather than cached at startup, enabling dynamic schema management without restarts
• New configuration-store crate with ConfigurationMode resolver and PostgresConfigurationStore backed by deadpool-postgres connection pool
• TLS support via native-tls for postgres connections (?sslmode=require in URL)
• Credential-safe Debug impls (postgres URL redacted in logs)
• Bumps Rust toolchain from 1.83.0 to 1.85.0 to fix CI build failure (wit-bindgen 0.51.0 requires edition2024)
• Updates nixpkgs and crane flake inputs to fix cargo-audit failure parsing CVSS 4.0 scores in newer RustSec advisories

Env vars

Variable	Purpose
HASURA_CONFIGURATION_MODE	json (default) or a postgres connection URL
HASURA_CONFIGURATION_CONNECTOR_ID	Required in postgres mode — scopes config queries
HASURA_CONFIGURATION_SCHEMA	Postgres schema name (default: connector_config)

Behavior in postgres mode

• /query: fetches schemas for all collections referenced in the request (primary + relationship targets) from config_tables.raw_schema, merges them into a single configuration
• /schema: returns empty response (schema managed externally)
• /mutation: returns 501 Not Implemented (native mutations not in postgres store)
• Connection URI: read from config_metadata table, falls back to MONGODB_DATABASE_URI env var

Changes in this update

• Relationship support: queries involving relationships (e.g. Album → Track → Genre) now work correctly. The connector extracts all collection names from the QueryRequest (primary collection + all collection_relationships targets) and fetches/merges their schemas from the config store.
• Schema fix: config_tables query uses is_deleted = false instead of deleted_at IS NULL to match the actual table schema.

Test plan

• All 145 existing unit tests pass
• All 27 connector-direct integration tests pass in postgres mode (grouping, filtering, nested collections, relationships, remote relationships, expressions)
• Relationship queries (joins, aggregates over related collections, groups through relationships) verified working
• Manual end-to-end test with local PostgreSQL + MongoDB containers
• Engine-dependent integration tests (require v3-engine, not tested locally)

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