Implement DBSIZE command

[NicoleStrel]

Implement the DBSIZE command, which returns the integer number of keys in the currently selected database.
Reference: https://redis.io/docs/latest/commands/dbsize/

Client-Server Spec:

Command File: ./internal/modules/generic/commands.go
Test File: ./internal/modules/generic/commands_test.go

Command: dbsize
Module: constants.GenericModule
Categories: constants.KeyspaceCategory, constants.ReadCategory, constants.FastCategory
Description: (DBSIZE) Return the number of keys in the currently selected database.
Sync: false

Embedded Spec:

Command File: ./echovault/api_generic.go
Test File: ./echovault/api_generic_test.go

Documentation

Add documentation to ./docs/docs/commands/generic/DBSIZE.mdx

[NicoleStrel]

^ will work on this