Partially adding sandbox metadata db

This PR adds the basic functionalities of SandboxMetadataDB. Partially addresses https://github.com/DIRACGrid/diracx/issues/13 . Few notes:

the DB schema added here is what is defined in https://github.com/DIRACGrid/DIRAC/pull/6566 (so, what is in DIRAC v8.1 but excluding OwnerDN).
Only insertion of a new SB is added, no other functionalities (deletion is tentatively added)
There are discussions going on in https://github.com/DIRACGrid/diracx/discussions/16 that might lead to a simplification of DIRAC's Sandbox (specifically if external Sandbox SEs are not supported) so I would wait before implementing further.

DIRACGrid / diracx