mgmtd: Bringup MGMTD daemon and db module support

mgmtd: Bringup MGMTD daemon and db module support

Features added in this commit:
1. Bringup/shutdown new management daemon 'mgmtd' along with FRR.
2. Support for Startup, Candidate and Running DBs.
3. Lock/Unlock DB feature using pthread lock.
4. Load config from a JSON file onto candidate DB.
5. Save config to a JSON file from running/candidate DB.
6. Dump candidate or running DB contents on the terminal or a file in
   JSON/XML format.
7. Maintaining commit history (Full rollback support to be added in
   future commits).
8. Addition of debug commands.

Co-authored-by: Yash Ranjan <ranjany@vmware.com>
Co-authored-by: Abhinay Ramesh <rabhinay@vmware.com>
Co-authored-by: Ujwal P <ujwalp@vmware.com>
Signed-off-by: Pushpasis Sarkar <pushpasis@gmail.com>