[parachain-template] benchmarks into `mod benchmarks` (#3818)

This PR introduces a dedicated module for benchmarks within the
parachain runtime. By segregating benchmarks into their own module, we
achieve a cleaner project structure and improved readability,
facilitating easier maintenance and updates.

### Key Changes:
- **New Benchmarks Module**: A new file `benchmarks.rs` is added,
encapsulating the benchmarking code for various pallets.
- **Refactoring `lib.rs`**: The main runtime library file (`lib.rs`) has
been updated to reflect the extraction of benchmark definitions. By
moving these definitions to `benchmarks.rs`, we reduce clutter in
`lib.rs`, streamlining the runtime's core logic and configuration.

### Benefits of This Refactoring:
- **Focused Benchmarking**: Developers can now easily locate and modify
benchmarks without navigating through the core runtime logic, enabling
targeted performance improvements.
- **Cleaner Codebase**: Segregating benchmarks from the main runtime
logic helps maintain a clean, well-organized codebase, simplifying
navigation and maintenance.
- **Scalability**: As the parachain evolves, adding or updating
benchmarks becomes more straightforward, supporting scalability and
adaptability of the runtime.

### Summary of Changes:
- Created `benchmarks.rs` to house the benchmarking suite.
- Streamlined `lib.rs` by removing the inlined benchmark definitions and
linking to the new benchmarks module.

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Co-authored-by: Kian Paimani <[email protected]>