Add DSCM NBI plugin along with RESTful API for DSCM pluggables configuration

- Implemented a file-backed JSON datastore for managing device configurations. (to be replaced with DSCM component)
- Created a DSCM plugin to handle device-specific configurations using NetConf.
- Added RESTful routes for CRUD operations on device configurations.
- Introduced header enforcement for content negotiation in API requests.
- Developed error handling responses for YANG data formats.
- Implemented path resolution for RESTCONF data paths.
- Added unit tests for the DSCM REST API, including GET, POST, PATCH, and DELETE operations.
- Included requirements for Flask and testing libraries in the requirements.txt.