crocket 0.3.0

Databases: crocket::db with its Postgres backend crocket::pg, and handlers that
return plain structs. Nothing existing breaks; the new pieces are opt-in.

Added

• crocket::db, a small database API, with its first backend crocket::pg
  (CMake target), Postgres on libpqxx, built when libpqxx is found
  (-DCROCKET_PG=AUTO|ON|OFF). Handlers take Db& and never name the backend. Plain structs, no annotations or base class: the
  table is the type's name in plural snake_case (Hero -> heroes, overridable with
  db::table_name<T>), columns are the members, id is the key. Handlers take Db& db;
  db.find<T>(id), db.insert(v), db.update(v), db.erase<T>(id),
  db.where(&T::member, value | db::like(...) | db::gt(...) | db::in({...})) with
  order_by, limit, offset, all, first and count, plus db.sql<T>(...) and
  db.exec(...) for raw SQL. Statements are prepared once per connection.
• A model's id may be missing from a JSON body (it reads as 0 and the database
  assigns it); insert() never sends id, so a client-supplied one is ignored.
  json::omittable<T> is the hook.
• crocket_db_seconds{table,op} and crocket_db_pool_connections{state} in
  Metrics, and db_ms, db_wait_ms and db_queries on Logger request lines,
  when a request used the database.
• Config::database (url, pool_size, pool_wait) and CROCKET_DATABASE_URL,
  CROCKET_DATABASE_POOL_SIZE and CROCKET_DATABASE_POOL_WAIT in Config::from_env().
• Handlers can return a plain struct: Hero, std::vector<Hero>, std::optional<Hero>
  (empty: 404) and Created<Hero> are JSON on HTTP routes, as if wrapped in Json<T>,
  and protobuf on gRPC routes. Json<T> still works and always wins.
• Proto<T>: an explicit protobuf body, application/x-protobuf on HTTP routes and
  the normal reply on gRPC routes. A gRPC method returning Json<T> replies with JSON
  (application/grpc+json). OpenAPI lists application/x-protobuf for Proto<T>.
• examples/heroes.cpp: a CRUD API on crocket::db, and bench/db.cpp, crocket::db
  against the same queries written with libpqxx.