Custom recurrence billing frequency type for the clients

[GauravGusain98]

Is your feature request related to a problem? Please describe.
In one of our projects, invoices are issued biweekly, which creates a challenge in capturing efforts for the project. Since efforts are tracked based on a monthly billing model, the billing dates calculated by the effort synchronization logic do not align with the actual invoice dates.

Describe the solution you'd like
To resolve this, on the client edit screen, we need to add an option to set the billing frequency to Custom Recurrence. When this option is selected, the billing day field will be hidden, and two new fields, Start Date and Repeat Every (in days), will be displayed. When determining the billing date for a client or project, we will apply the appropriate logic based on the selected billing frequency.

Additional context

1. For calculating the billing start date and end date we have created Laravel accessors in Client Model with name getMonthEndDateAttribute and getMonthEndDateAttribute. We just need to make some modification in these accesors to get the correct start date and end date for the billing period