[Feature] Add DEBUG_DROP_MONITOR table in schema.h

[arista-hpandya]

This issue tracks the changes made to schema.h to add support for DEBUG_DROP_MONITOR table. This is one of the many changes needed to implement the configurable drop counter monitoring feature tracked here: sonic-net/SONiC#1912

Changes to be made:

• Add DEBUG_DROP_MONITOR table in common/schema.h

Additional Details:
This change is a pre-requisite for sonic-net/sonic-swss#3509, which implements the core logic behind the feature.

References:

• Related HLD document: HLD document for configurable drop counter monitoring SONiC#1912
• Master issue tracking overall progress: Internal drop counter monitoring SONiC#1542