Add MySQL metastore backend

Description

Adds MySQL as a new production-grade metastore backend for Quickwit, enabling deployments that prefer MySQL over PostgreSQL for storing index metadata, splits, shards, delete tasks, and index templates.

The existing metastore has two backends: file-backed (dev) and PostgreSQL (production). This PR introduces a third: MySQL, following the same architecture and feature surface as the PostgreSQL backend.

MySQL metastore modules

The MySQL metastore lives in quickwit-metastore/src/metastore/mysql/ and mirrors the PostgreSQL metastore module-for-module:

Module	Purpose
metastore.rs	Full MetastoreService trait implementation (~2,158 lines) covering all 30+ metastore operations: index CRUD, split staging/publishing/deletion, shard open/acquire/prune, delete tasks, and index templates.
model.rs	Row types (MysqlIndex, MysqlSplit, MysqlShard, MysqlDeleteTask, MysqlIndexTemplate) for SQLx row deserialization.
factory.rs	MysqlMetastoreFactory — URI-based factory that creates a MysqlMetastore from a mysql:// connection string.
pool.rs	TrackedPool<MySql> — connection pool wrapper with metrics tracking for active/idle connections.
migrator.rs	Schema migration runner with configurable locking (skip via QW_MYSQL_SKIP_MIGRATION_LOCKING).
error.rs	SQLx error → MetastoreError conversion, mapping MySQL error codes to domain errors.
tags.rs	Tag filter → SQL WHERE clause generation using sea_query.
utils.rs	Query building helpers: split filters, maturity timestamps, column selection, ordering.
split_stream.rs	Streaming split results in chunks (STREAM_SPLITS_CHUNK_SIZE).
metrics.rs	MySQL-specific metastore operation metrics.
auth.rs	New (not in Postgres) — AWS IAM authentication token generation for RDS MySQL.
queries/	Raw SQL files for shard and index template operations using MySQL syntax (ON DUPLICATE KEY UPDATE).

Protocol & type layer changes (quickwit-common, quickwit-proto)

Change	Why
Added Protocol::MySQL variant with mysql:// URI parsing	Enables mysql://user:pass@host:3306/db metastore URIs with credential redaction in logs.
is_database() now covers Protocol::MySQL	Prevents invalid operations like join() and parent() on database URIs.
Added sqlx::Type<MySql> + sqlx::Encode<MySql> for IndexUid, DocMappingUid, ShardId	SQLx requires per-database-driver trait impls — these cannot be shared with the Postgres impls (see rationale below).
Broadened #[cfg(feature = "postgres")] → #[cfg(any(feature = "postgres", feature = "mysql"))]	Shared code like sea_query::Value conversions and TryFrom<String> impls are needed by both backends.

Configuration (quickwit-config)

Change	Why
MetastoreBackend::MySQL enum variant	Enables metastore_backend: mysql in config files.
MetastoreConfig::MySQL(MysqlMetastoreConfig)	Holds connection pool settings (min_connections, max_connections, acquire_timeout, idle_timeout, max_lifetime).
MysqlAuthMode enum (Password, AwsIam)	Supports both password-based and IAM token-based authentication for AWS RDS.

Schema (migrations/mysql/)

1_initial-schema.up.sql creates six tables adapted for MySQL's DDL dialect:

• indexes — index metadata with index_metadata_json column (TEXT)
• splits — split lifecycle management with BIGINT timestamps, split_metadata_json (TEXT), and JSON tag columns
• shards — shard state tracking with publish_position_inclusive (VARCHAR(255))
• delete_tasks — delete query queue with delete_query_json (TEXT)
• index_templates — index template storage with index_template_json (TEXT) and priority-based matching
• kv — key-value metadata storage

Resolver & feature gating

• MysqlMetastoreFactory registered in MetastoreResolver behind #[cfg(feature = "mysql")]
• UnsupportedMetastore fallback when compiled without the feature (matches Postgres pattern)
• mysql feature added to quickwit-metastore, quickwit-proto, and quickwit-cli Cargo.toml

Docker & testing infrastructure

• MySQL 8.0 service added to docker-compose.yml with health checks
• .env.example updated with QW_MYSQL_* connection variables

How was this tested?

• MySQL 8.0 Docker service with health checks for local testing
• Unit tests added for MySQL-specific functionality
• Migration up/down scripts verified against fresh MySQL instances
• URI parsing tests for mysql:// protocol including credential redaction
• Feature-gated compilation verified with and without the mysql feature
• Currently running in our K8s env connected to RDS

Couldn't find any contribution rules about LLM usage but warning for substantial LLM usage in this PR!