feat: implement preliminary composite primary key support

Add support for entities with composite primary keys using multiple
#[georm(id)] fields. Automatically generates {EntityName}Id structs for
type-safe composite key handling.

Features:
- Multi-field primary key detection and ID struct generation
- Full CRUD operations (find, create, update, delete, create_or_update)
- Proper SQL generation with AND clauses for composite keys
- Updated documNtation in README and lib.rs

Note: Relationships not yet supported for composite key entities

Cargo.toml

@@ -39,8 +39,14 @@ sqlx = { workspace = true }
 georm-macros = { workspace = true }
 
 [dev-dependencies]
+chrono = { version = "0.4", features = ["serde"] }
 rand = "0.9"
 
+[dev-dependencies.sqlx]
+version = "0.8.6"
+default-features = false
+features = ["postgres", "runtime-tokio", "macros", "migrate", "chrono"]
+
 [workspace.lints.rust]
 unsafe_code = "forbid"