Implement DECRBY command

[kelvinmwinuka]

Implement the DECRBY command, which decrements the value at the specified key by the specified amount if the value is an integer or a float.
Reference: https://redis.io/docs/latest/commands/decrby/

Client-Server Spec:

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

Command: decrby
Module: constants.GenericModule
Categories: constants.WriteCategory, constants.FastCategory
Description: (DECRBY key decrement) The DECRBY command reduces the value stored at the specified key by the specified decrement. If the key does not exist, it is initialized with a value of 0 before performing the operation. If the key's value is not of the correct type or cannot be represented as an integer, an error is returned.
Sync: true

Embedded Spec:

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

[dotslashbit]

working on it