Feature: Add Note to Budget Command

[Patmut254]

The backend development approach used includes:

1. CQRS pattern with separate command and handler.
2. AddNoteToBudgetCommand class which encapsulates the data required to be added a note.
3. AddNoteToBudgetHandler class that performs validation and calls repository methods.

The design choices used in the backend development are:

1. A validation which ensures budgetId and content exist.
2. Dependencies like getRepository come from the FunctionHandler toolkit for decoupled data access.
3. A chose void return type for compatibility with ICommandHandler interface.

Finally, deployment and serverless for the project:

1. A handler which is ready for serverless deployment via @ngfire/functions.
2. Scales horizontally which each request invokes a new function instance, handling high concurrency efficiently.