[parachain-template] runtime API Implementations into `mod apis` (#3817)

This PR significantly refactors the runtime API implementations to
improve project structure, maintainability, and readability. Key changes
include:

1. **Enhancing Visibility**: Adjusts the visibility of
`RUNTIME_API_VERSIONS` in `impl_runtime_apis.rs` to `pub`, making it
accessible throughout the runtime module.
2. **Centralizing API Implementations**: Introduces a new file,
`apis.rs`, within the parachain template's runtime directory.
3. **Streamlining `lib.rs`**: Updates the main runtime library file to
reflect these structural changes. It removes redundant API
implementations and points `VERSION` to the newly exposed
`RUNTIME_API_VERSIONS` from `apis.rs`, simplifying the overall runtime
configuration.

### Motivations Behind the Refactoring:
- **Improved Project Structure**: Centralizing API implementations in
`apis.rs` offers a clearer, more navigable project structure.
- **Better Readability**: Streamlining `lib.rs` and reducing clutter
enhance readability, making it easier for new contributors to understand
the project layout and logic.

### Summary of Changes:
- Made `RUNTIME_API_VERSIONS` public in `impl_runtime_apis.rs`.
- Added `apis.rs` to centralize runtime API implementations.
- Streamlined `lib.rs` to adjust to the refactored project structure.